Analytic geometry & the continuum (56:34) N.J. Wildberger (2011年05月09日).
For a point (x,y) on that line, the vector from (-8,9) to (x,y) is proportional to the vector from (-8,9) to (10,-3). The relevant determinant vanishes:
Thus, 18(y-9) + 12(x+8) = 0. which boils down to the equation of the line:
2 x + 3 y = 11
Ivory's Theorem: In a curvilinear quadrangle formed by two pairs of confocal conics, the diagonals are equal.
[画像: Come back later, we're still working on this one... ]
The conic sections were named by Apollonius of Perga (262-190 BC).
Dandelin spheres (1822)
are due to
(Pierre) Germinal Dandelin (1794-1847; X1813)
partial credit also goes to
Adolphe Quételet (1796-1874)
Hamilton (1805-1865)
remarked that a Dandelin sphere touches the cone in a plane containing a directrix.
In 1829,
Pierce Morton (1803-1859)
used Dandelin spheres to prove the focus-directrix property of conics.
A spiral of Archimedes may be described as the trajectory of a point which moves at constant speed along a spoke of a wheel rotating at a constant rate.
As the distance (r) from the center to a point on the spiral is proportional to the angle (q) between the spoke and some fixed line, the polar equation is:
r = a q
The angle q (expressed in radians) increases by 2p with each turn. Therefore, the quantity 2pa is the thickness of the paper on a roll, or the distance between adjacent parts of the groove on a vinyl record.
When the polar coordinates (r,q) of a point change minutely by (dr,dq), that point travels a minute distance ds, given by the following relation (obtained from the Pythagorean theorem, since a smooth curve looks straight at small enough scales):
(ds)2 = (dr)2 + (r dq)2
In the case of Archimedes' spiral, this means: ds = a (1 + q 2 )½ dq. The length (s) of the spiral is obtained by integrating this:
For a roll of paper of diameter 2r1 with a core of diameter 2r0 the total length is simply s(r1)-s(r0). [Recal that 2pa is the thickness of the paper.]
The exact formula above is an overkill for estimating the length of paper on a roll. Instead, you may consider that, if the paper is very thin compared to the diameter of the core, the surface area of the roll's cross section divided by the thickness (e = 2pa) of the paper gives you its length, namely:
s » (r12 - r02 ) / 2a = p (r12 - r02 ) / e
This approximation is just the leading term of the above exact formula.
For a 65 ft roll of paper towels, 2r1 = 5" and 2r0 = 1.75".
The thickness of the paper, measured with a caliper, is 0.01". Plugging this into the formula yields a length of about 144 ft, which is more than twice the actual value. What's wrong?
A paper towel normally has features (fluffiness, waviness, embossment) which make the caliper measurement of a single sheet greatly underestimate the effective thickness of the material in a stack or a roll. By working the formula backwards, the above numbers would indicate that the paper material has an effective thickness of about 0.022". This could be obtained directly by counting the number N of turns it takes for the diameter of the roll to decrease by x inches (the effective thickness of the stuff would then be x / 2N). If you insist on using a caliper, you may reduce by a factor of N the large error obtained for a single sheet, simply by using the caliper (without squeezing) on a stack of N sheets...
If a roll of paper towels of the type quoted above has a diameter D (in inches) then its footage is approximately: ( D 2 - 3.06 ) 2.96 ft
r = a / q
A thin uniform rope without any rigidity is traditionally called a chain (in Latin: catena). The lack of any force sideways means there's just a tangential tension. Let's characterize the shape (catenary) of a chain under its own weight:
Let j be the inclination of the chain (i.e., the angle its tangent makes with the horizontal x-axis) at a given curvilinear abscissa s. The tension pulling on the left part of the chain has magnitude F and coordinates (F cos j, F sin j). For a positive displacement ds along the rope, the infinitesimal increase of the vertical component is balanced by the weight of the corresponding piece of rope (we assume the mass per unit of length to be a constant m ) :
d ( F sin j ) = g m ds
As the horizontal component of the tension is a constant: F = K / cos j
d ( K tan j ) = g m ds
A Cartesian relation is readily obtained by using y' = dy/dx = tan j
K d(y' ) = g m ( 1 + (y' ) 2 ) ½ dx
This is integrated as a simple quadrature, by considering x as function of y' :
x = xo + a Argsh ( y' ) [ introducing a = K/gm ]
Conversely, y' = sh [(x-xo )/a]. Therefore, by integration:
y = yo + a ch [(x-xo )/a]
Thus, if we use the apex of the curve as origin of the coordinates, we have:
The curvilinear abscissa is simply : s = a sh (x/a) = a tan j
For a plane curve, such a relation between the arclength s and the inclination j goes by the name of Whewell equation. It can be used to derive the so-called intrinsic Cesàro equation which relates the (signed) radius of curvature R = ds/dj and the arclength s. In the case of the catenary, we have:
R = ds/dj = a ( 1 + tan2 j ) = a + s2/a
So, the parameter a is the radius of curvature at the catenary's apex (s=0).
Galileo (1564-1642) thought that a chain under its own weight would take on a parabolic shape, which Jungius (1587-1657) disproved. The latin name catenaria was coined by Huygens in 1690 (in a letter to Leibniz). This was immediatly translayed as "chainette" in French. In English, the locution "catenarian curve" was used at first. Arms of Thomas Jefferson Thomas Jefferson introduced the word catenary a century later (1788). The above equations for a catenary were first derived in 1691, by Leibniz, Huygens and Johann Bernoulli, in their respective responses to an open challenge from Jakob Bernoulli.
The Catenary - Mathematics all around us, by James Grime (singingbanana). | Wikipedia : Catenary
The evolute of the tractrix is the catenary.
The surface generated by revolving the tractrix about its asymptote is known as the tractricoid. It has a finite surface area of 4pa 2 and it encloses a finite volume of 2/3 pa 3.
The tratricoid is a surface of constant negative Gaussian curvature. Such a surface is known as a pseudosphere and provides a concrete model for the hyperbolic geometry first described axiomatically (negating Euclid's fifth postulate) by Lobachevsky and Bolay (Beltrami, 1868).
MacTutor | Wikipedia : Tractrix | Tractricoid
The curve's cartesian equation is: y = a3 / (x2+a2 ). It's associated with a circle of diameter a tangent to the x-axis at the origin O. A point M on this curve may be obtained via a line OPQ where P is on the circle and Q is at height a; M has the x-coordinate of Q and the y-coordinate of P.
This curve was first considered by: Maria Gaetana Agnesi (1718-1799) Pierre de Fermat (1601-1665)
It seems you've got to be about 30 years of age to be interested in this... Just a joke!
The Italian name versiera (used by Agnesi) was actually given to the curve by Grandi in 1718, because of the "inverted sine" (sinus versus) which is used in its construction. In spite of what is often stated, there may not be any direct relation with the Latin word versoria which denotes the rope used on a sailboat to transfer edge (the verb vertere means "to turn"). The weird name of "witch" comes from a famous mistranslation by the Lucasian professor of mathematics at Cambridge, John Colson (1680-1760). When he translated Maria Agnesi's Istituzioni Analitiche [written in 1748, this was the first calculus book ever authored by a woman] Colson apparently mistook "la versiera" for "l'avversaria"... Since l'avversario (the adversary) is the Devil, an avversaria would be a she-devil, or a witch! Colson's work seems also responsible for the attribution of the curve to Agnesi (she made no such claim herself).
The area between the versiera and its asymptote [the x-axis] is pa2, which happens to be exactly 4 times the area of the associated circle.
The function a / p(x2+a2 ) thus corresponds to the density of a continuous probability distribution (whose standard deviation is infinite ).
Its cartesian equation is: x 3 + y 3 = 3axy
The asymptote is of equation:
x + y + a = 0
The surface area of the lobe is 3 a 2/ 2
. So is the area between the asymptote and the curve.
The total signed area bounded by the oriented closed loop consisting of the folium and its asymptote is zero (that loop is understood to include 2 points "at infinity" and to have a continuous tangent vector at the double point).
The Latin word "lemniscus" means pendant ribbon. This name was first used in 1694 by Jacob Bernoulli (1655-1705) to describe the above planar curve, which is now named after him.
A lemniscate of Bernoulli is the particular Cassinian oval which goes through the midpoint between its foci. If 2a is the distance between the foci, the lemniscate's cartesian equation in the proper coordinate system is:
( x 2 + y 2 ) 2 = 2 a 2 ( x 2 - y 2 )
The horizontal width of the curve is 2Ö2 a, while its height (or vertical width) is simply a. The surface area of each lobe happens to be simply a2.
A multipolar Cassini ovum can be defined as the set of all points whose distances to a certain number of fixed poles have a constant product.
In the plane, such sets are curves (circles in the monopolar case, traditional ovals of Cassini in the dipolar case). In 3-dimensional Euclidean space, they are 2-dimensional surfaces (possessing cylindrical symmetry in the dipolar case only). A Cassini ovum in higher dimensions is an hypersurface.
The above generalized definition isn't universally accepted. It's prudent to restate it before use, possibly with a link to this note:
These curves are named after Etienne Pascal (1588-1651), the father of the French scientist and philosopher Blaise Pascal (1623-1662). Etienne Pascal was part of a circle of mathematicians who held weekly meeting in Paris, around Marin Mersenne (1588-1648).
[画像:Limacons of Pascal] This famous mathematical group included the likes of Pierre Hérigone (1580-1643), Claude Mydorge (1585-1647), Girard Desargues (1591-1661), Pierre Gassendi (1592-1655), Jean Beaugrand (1595-1640), René Descartes (1596-1650), Claude Hardy (1598-1678), Pierre de Carcavi (1600-1684) and also Gilles Personne de Roberval (1602-1675) who is the one who actually named the limaçon after Etienne Pascal in 1650, when he used it as an example for the drawing of tangents.
A limaçon can probably be defined in more different ways than any other quartic curve. It can be described in any of the following equivalent ways:
A cardioid is a limaçon for which a = b. A circle is obtained for b = 0.
Given two poles A and B, a cartesian oval is the set of all points M for which AM + l BM is a constant (for some fixed parameter l).
[画像:Ovals of Descartes]This three-parameter family of curves includes the above [two-parameter] limaçons of Pascal as a special case (corresponding to the cartesian ovals which go through a pole).
If the cartesian equation of each curve is F(x,y,t) = 0 for a fixed value of the continuous parameter t, then the equation of the envelope is obtained by eliminating t between the two equations:
F = 0 and ¶F / ¶t = 0
Christiaan Huygens (1629-1695) A concept due to Huygens (1673). The tangents of the evolute are normal to the curve. A smooth curve is the evolute of any involute.
The evolute is also the locus of the curve's centers of curvature.
Evolute | Christiaan Huygens (1673)
Christiaan Huygens (1629-1695) A concept due to Huygens (1673). The normals of the involute are tangent to the curve. A smooth curve is an involute of its evolute.
The involute can also be envisioned as the trajectory of the extremity of a taut string that is wound onto (or unwound from) the curve.
If you must, use this latter viewpoint piecewise for parts of the curve whose curvature has a constant sign. There's no mathematical difficulty when the curve crosses its tangent, but then it can't really be viewed as the perimeter of an actual spool of string.
Of particular interest is the involute of a circle (which is what the term "involute" is understood to denote when it doesn't come with a qualifier). This is the only tooth profile that makes a driven spur gear rotate at a uniform rate (or "steady speed") when the driver does. All other types of spur gear will thus induce systematic vibration that are unacceptable for high-speed heavy machinery (but perfectly fine for low-speed clockwork in which pulsed rotation is the norm).
Involute | Christiaan Huygens (1673)
The envelope of the circles of radius a centered on a curve G is generated by the two points at distance a on the normal to G at a running point P. For a simple closed curve, this envelope can be divided into two separate curves; the inner and outter parallel at offset a, which are respectively generated by the points on the normal which lay on the inward or outward side of the normal. (In the field of computer graphics, parallel cfrom each other by translation.
The perimeters of a smooth closed curve and that of one of its [single-sided] parallels at offset a have either a sum or a difference equal to 2pa.
This curve was introduced in 1678 as the catacaustic of a circle by Christiaan Huyghens (1629-1695) who would later discuss it in Traité de la lumière (1690).
The term nephroid was coined by the English astronomer Richard A. Proctor (1837-1888) in The Geometry of Cycloids (London, 1878).
x = a ( 3 cos t - cos 3t )
y = a ( 3 sin t - sin 3t )
Freeth's nephroid was first described (with other strophoids) by the vicar Thomas Jacob Freeth (1819-1904) in 1879 (London Mathematical Society).
r = a ( 1 + 2 sin q/2 )
Bézier curves are often restricted to planar geometry, but they need not be.
Pierre Bézier (1910-1999) served as an engineer for the French car manufacturer Renault for 42 years, from 1933 to 1975. He was also a professor at the Conservatoire national des arts et métiers (CNAM) from 1968 to 1979. He introduced the type of curves now named after him for "free-form curves" within the CAD/CAM system dubbed UNISURF, which he launched in 1968.
The order-n Bézier curve from P0 to Pn involves n+1 control points (Pi ). A point M(t) of the curve is the following barycenter of the control points:
Unless otherwise specified, order n = 3 is understood, in which case alternate names may be used: "Bézier cubic", "cubic spline" (or just "spline"). Such a curve is given by its extremities A and B and two other control points P and Q:
M(t) = (1-t) 3 A + 3 t (1-t) 2 P + 3 t 2 (1-t) Q + t 3 B
[画像: Bezier curve ]As t goes from 0 to 1, M(t) goes from A to B. Assuming distinct control points, the line AP is tangent to the spline at point A, whereas BQ is the tangent at point B.
It's good to know that if PQ is parallel to AB and
PQ = AB/3, then the Bézier cubic spline is actually a
parabolic arc.
Thus, it's easy to obtain parabolas from any computer graphic package that provides
Bézier cubics (most of them do).
Bézier curves of order 2 (with a single intermediary control point R)
[画像: Quadratic Bezier Curve ]
correspond to these same parabolic arcs,
since the relations
P = (A+2R)/3 and
Q = (2R+B)/3 imply:
(1-t) 3 A
+
3 t (1-t) 2 P
+
3 t 2 (1-t) Q
+
t 3 B
=
(1-t) 2 A
+
2 t (1-t) R
+
t 2 B
Any Bézier curve of order n (with control points Pi ) is also a Bézier curve of order n+1 with control points Qi given by the following relation. Note that: Q0 = P0 and Qn+1 = Pn [HINT: Expand (1-t) M(t) + t M(t) ]
Pascal's formula makes this a barycentric relation: Qi is between Pi-1 and Pi (e.g., in the above parabolic-to-cubic case, P is on AR and Q is on RB).
In the computer era, the most popular approximations for arbitrary curves involve algebraic splines, like the Bézier curves discussed above. For more traditional work, the circle remains, indeed, the basic curve of choice.
The following procedure assumes that the curve we want to approximate contains no straight sections and no angular points (otherwise we'd process separately each piece of the curve between these).
To build a smooth curve consisting of circular arcs which approximates the original curve, we first choose a number of "construction points" located on that curve. If the curve is open, we must include its extremities among these. It's also a good idea to include any inflection points the original curve may have. (Inflection points are points where the curve crosses its own tangent. Near one of its inflection points, the curve is usually very close to a straight line.)
Such lines always intersect if all inflections points have been included among the construction points; parallelism would indicate that we're bracketing an inflection point and need more intermediary construction points. (A practical alternative is to "cheat" by introducing small straight sections around inflection points.)
The intrinsic equation of a piecewise circular function is a step function...
The curvilinear abscissa (often denoted s) is the arc length measured along the curve from some arbitrary origin on it (counted negatively "before" this origin).
At a particular point, the curvature of a planar curve has a magnitude equal to the reciprocal of its radius of curvature (the radius of the so-called osculating circle which best approximates the curve in the immediate neighborhood). By convention, the sign of the curvature is positive if the curves turns to the left of the direction of an increasing curvilinear abscissa. The curvature is zero at an inflection point, or at any point of a straight section.
The so-called intrinsic equation (which gives the curvature of the curve as a function of the curvilinear abscissa along it) uniquely defines the "absolute" shape of a planar curve (it could be translated or rotated, but not flipped over).
The above viewpoint is only adequate for smooth curves with no angular points. However, it's easy to generalize it to angular curves, since the curvature is the derivative [with respect to curvilinear abscissa] of the angle (in radians) between some fixed direction and the tangent to the curve. A sharp turn of q radians at a point so of infinite curvature thus translates into a Dirac spike q d(s-so) in the intrinsic equation (namely, a Delta distribution of amplitude q ).
The quadratrix is the trajectory of the intersection of two straight lines: One in uniform motion, the other uniformly rotating about a fixed point A.
Infinitely many constructible angles can be used to obtain points of the quadratrix. In particular, as any angle can be bissected with straightedge and compass, we may divide a circle in 2, 4, 8, 16, 32, 64... 2 n equal angles. Draw such radial lines for a sufficiently large n, then draw any set of equally-spaced parallel lines. The points where successive radial lines intersect successive parallel lines are on a quadratrix... (Intermediary points may be obtained by doubling up the lines in both sets.)
The ancient Greeks only considered those points which could be deduced from a few given points in finitely many uses of a straightedge or a compass. The quadratrix of Hippias was the first curve on record whose points cannot all be so deduced. Thus, it's not considered a constructible curve, although infinitely many points on it can be constructed classically, as shown above.
In the proper coordinate system, the cartesian equation of the quadratrix is:
y = a - x / tan(x/a)
The center of rotation (A) is at (x,y) = (0,a). The curve's apex (O) is at (0,0). The main branch is bounded by two vertical asymptotes (-p < x/a < p) and corresponds to a full turn of the rotating line. Only the central portion is usually considered (-p/2 < x/a < p/2) which corresponds to a half-turn of the rotating line, between horizontal positions. The portion where x is positive may suffice.
The curve was devised around 420 BC by Hippias of Elis. It's been called either quadratrix or trisectrix because it provides graphic solutions to two famous classical problems, which are now known to be outside the scope of what can be done with straightedge and compass: The squaring (quadrature) of the circle and the trisection of an arbitrary angle.
Also, Heath has attributed to Archytas of Tarentum a "quadratrix solution" to the third most famous classical problem of this kind, the duplication of the cube, or Delian problem. Legend has it that the Delian problem dates back to 430 BC, when the Oracle of Apollo at Delos said the plague ravaging Athens would come to an end if the altar of Appollo could be made exactly "twice as large" [by volume]. The cube root of 2 is still occasionally known as the Delian constant.
The quadratrix can be used to "square the circle" because the number p (which can't be constructed with straightedge and compass) is simply obtained as the ratio of the width to the height of the section of the quadrarix whose extremities are on an horizontal line through A.
To obtain a portion l of an arbitrary angle (including l = 1/3) whose vertex is at point A, consider the two points where its sides intersect the main branch of the quadratrix. Draw trough them 2 lines parallel to the axis of the quadratrix. If M is the point where the quadratrix intersects a third parallel line dividing the space between them in the ratio l, then the line AM divides our angle in the ratio l.
This wonderful curve is thus directly related to yet another famous problem: the construction of an n-sided regular polygon. Equilateral triangles, squares, regular pentagons and hexagons are all constructible with straightedge and compass, but it turns out that regular heptagons, enneagons, hendecagons, or tridecagons cannot be so obtained. This was established in 1796 by Carl Gauss (at age 19) who showed the heptadecagon (17 sides) to be classically constructible.
A parabola is the set of all points of the plane at an equal distance from a given point (called the parabola's focus) and a given line (the parabola's directrix).
The parabola is called "constructible" because its intersection with an arbitrary straight line, or an arbitrary circle, can always be located in finitely many steps using only straightedge and compass. Thus, allowing the use of of a parabolic spline in addition to straightedge and compass would not introduce any additional "constructible" points. This could make classical geometry easier, but not richer.
There is only One True Parabola by Matt Parker (2016年03月03日).
Georg Mohr had also shown that all Euclidean constructions could be done with only a straightedge, if a circle of given center is already drawn somewhere in the plane. The Swiss mathematician Jakob Steiner (1796-1863) rediscovered this in 1833 (Steiner constructions).
We may prove that all constructible points can be obtained with the compass alone by providing Mohr-Mascheroni constructions for the two types of points which normally involve the use of the straightedge:
We'll do so using only two explicit Mohr-Mascheroni constructions, concerning the inversion with respect to a circle and the circumcenter of three points. The very simple construction we'll provide for the former isn't completely general unless supplemented by a scaling-up procedure, like the following one:
An example of a Mohr-Mascheroni construction is the doubling, which we'll use below: Given two points O and M, the point N in the direction of OM such that ON = 2OM may be obtained as follows:
Draw the entire circle (C) of center M and radius OM.
It intersects the circle of center O and radius OM at points A and B.
The circle of center A through B (or center B through A) intersects (C)
at point N Halmos
(It wouldn't be acceptable to define N as the intersection of the circles centered on A and B, because there are two such points of intersection, both "new".)
The so-called inversion of the Euclidean plane with respect to a circle of center O and radius R is a transformation which was introduced in 1826 by Steiner (in his first published paper). The inverse of a point M (other than O) is the point N in the direction of OM which is such that OM.ON = R2. This transforms a circle through O into a straight line, and vice-versa. Circles not containing O are simply associated with other such circles.
[画像: Inversion ]N is very simple to obtain from M with the compass alone: Assume OM is more than R/2 and consider the intersections A and B of the invariant circle with the circle of center M and radius OM. Let N be the intersection (besides O) of the circles of centers A and B going through O. We have:
OM . ON = R 2
This relation is easy to prove with cartesian coordinates: O=(0,0), A=(x,y), N=(2x,0), M=(a,0). Since, R=OA and MO=MA, we have: R2 = x2 + y2 and a2 = (a-x)2 + y2. By subtraction: R2 = 2xa = OM.ON Halmos
If OM is R/2 or less, we may first obtain a point M' such that OM' = 2n OM, using the above doubling n times, for a value of n large enough to make OM' greater than R/2. The inverse N' of M' is then constructed as above. Finally, the inverse N of M is simply obtained by doubling ON' n times.
The construction, with straightedge and compass of the center of the circle circumscribed to a triangle is a very early piece of mathematics; the circumcenter of a triangle is the intersection of the perpendicular bissectors of two sides. The following Mohr-Mascheroni construction uses an inversion centered on one of the 3 vertices to obtain the inverse of the circumcenter as the intersection of the two circles which are inverses of these bissectors.
Consider a triangle ABC, whose circumcenter N is sought. In an inversion of center A and radius AB, the image of the perpendicular bissector of AB is a circle U of center B and radius AB. On the other hand, the inverse [with respect to the circle U] of the perpendicular bissector of AC is a circle V of diameter AH', where H' the inverse of the middle H of AC. The center of V is thus the inverse C' of C (constructed as indicated above). The circles (U and V) through O of centers B and C' intersect at another point M, whose inverse (with respect to U) is the circumcenter N of ABC Halmos
Of course, once the circumcenter is thus obtained, with compass alone, the entire circle through A, B and C is easily drawn...
In classical geometry, any planar figure consists of straight lines and circles:
Consider an inversion of radius R whose center O is distinct from the finitely many points of intersection of such a classical figure (it's best to let the distance of O to all such points be between R/2 and R so the simplest compass construction can be used to go back and forth between a point and its image):
These remarks and the above explicit compass-only constructions allow the inverse of all the elements of a Euclidean planar figure to be drawn with compass alone. The intersections of such elements can be constructed with compass alone as the inverses of the intersections in the inversed figure. Halmos
Tait-Kneser Theorem (1896, 1912)
|
Peter Tait (1831-1901)
|
Adolf Kneser (1862-1930)
Osculating curves by
Etienne Ghys,
Serge Tabachnikov,
Vladlen Timorin (2012年07月24日).
