#Mathematica, 9 bytes
#(#+1)/2&
#Mathematica, 10 bytes
(#^2+#)/2&
#Mathematica, 11 bytes
Tr@Range@#&
#Mathematica, 12 bytes
i~Sum~{i,#}&
#Mathematica, 14 bytes
(by @user71546)
1/2/Beta[#,2]&
#Mathematica, 15 bytes Tr[#&~Array~#]&
#Mathematica, 16 bytes Binomial[#+1,2]&
#Mathematica, 17 bytes
(by @Not a tree)
⌊(2#+1)^2/8⌋&
#Mathematica, 18 bytes
PolygonalNumber@#&
#Mathematica, 19 bytes
#+#2&~Fold~Range@#&
#Mathematica, 20 bytes
(by @Not a tree)
f@0=0;f@i_:=i+f[i-1]
#Mathematica, 9 bytes
#(#+1)/2&
#Mathematica, 10 bytes
(#^2+#)/2&
#Mathematica, 11 bytes
Tr@Range@#&
#Mathematica, 12 bytes
i~Sum~{i,#}&
#Mathematica, 15 bytes Tr[#&~Array~#]&
#Mathematica, 16 bytes Binomial[#+1,2]&
#Mathematica, 17 bytes
(by @Not a tree)
⌊(2#+1)^2/8⌋&
#Mathematica, 18 bytes
PolygonalNumber@#&
#Mathematica, 19 bytes
#+#2&~Fold~Range@#&
#Mathematica, 20 bytes
(by @Not a tree)
f@0=0;f@i_:=i+f[i-1]
#Mathematica, 9 bytes
#(#+1)/2&
#Mathematica, 10 bytes
(#^2+#)/2&
#Mathematica, 11 bytes
Tr@Range@#&
#Mathematica, 12 bytes
i~Sum~{i,#}&
#Mathematica, 14 bytes
(by @user71546)
1/2/Beta[#,2]&
#Mathematica, 15 bytes Tr[#&~Array~#]&
#Mathematica, 16 bytes Binomial[#+1,2]&
#Mathematica, 17 bytes
(by @Not a tree)
⌊(2#+1)^2/8⌋&
#Mathematica, 18 bytes
PolygonalNumber@#&
#Mathematica, 19 bytes
#+#2&~Fold~Range@#&
#Mathematica, 20 bytes
(by @Not a tree)
f@0=0;f@i_:=i+f[i-1]
#Mathematica, 9 bytes
#(#+1)/2&
#Mathematica, 10 bytes
(#^2+#)/2&
#Mathematica, 11 bytes
Tr@Range@#&
#Mathematica, 12 bytes
i~Sum~{i,#}&
#Mathematica, 15 bytes Tr[#&~Array~#]&
#Mathematica, 16 bytes Binomial[#+1,2]&
#Mathematica, 17 bytes
(by @Not a tree)
⌊(2#+1)^2/8⌋&
#Mathematica, 18 bytes
PolygonalNumber@#&
#Mathematica, 19 bytes
#+#2&~Fold~Range@#&
#Mathematica, 20 bytes
(by @Not a tree)
f@0=0;f@i_:=i+f[i-1]
#Mathematica, 9 bytes
#(#+1)/2&
#Mathematica, 10 bytes
(#^2+#)/2&
#Mathematica, 11 bytes
Tr@Range@#&
#Mathematica, 12 bytes
i~Sum~{i,#}&
#Mathematica, 15 bytes Tr[#&~Array~#]&
#Mathematica, 16 bytes Binomial[#+1,2]&
#Mathematica, 18 bytes
PolygonalNumber@#&
#Mathematica, 19 bytes
#+#2&~Fold~Range@#&
#Mathematica, 20 bytes
(by @Not a tree)
f@0=0;f@i_:=i+f[i-1]
#Mathematica, 9 bytes
#(#+1)/2&
#Mathematica, 10 bytes
(#^2+#)/2&
#Mathematica, 11 bytes
Tr@Range@#&
#Mathematica, 12 bytes
i~Sum~{i,#}&
#Mathematica, 15 bytes Tr[#&~Array~#]&
#Mathematica, 16 bytes Binomial[#+1,2]&
#Mathematica, 17 bytes
(by @Not a tree)
⌊(2#+1)^2/8⌋&
#Mathematica, 18 bytes
PolygonalNumber@#&
#Mathematica, 19 bytes
#+#2&~Fold~Range@#&
#Mathematica, 20 bytes
(by @Not a tree)
f@0=0;f@i_:=i+f[i-1]
#Mathematica, 9 bytes
#(#+1)/2&
#Mathematica, 10 bytes
(#^2+#)/2&
#Mathematica, 11 bytes
Tr@Range@#&
#Mathematica, 12 bytes
i~Sum~{i,#}&
#Mathematica, 15 bytes Tr[#&~Array~#]&
#Mathematica, 16 bytes Binomial[#+1,2]&
#Mathematica, 18 bytes
PolygonalNumber@#&
#Mathematica, 19 bytes
#+#2&~Fold~Range@#&
#Mathematica, 20 bytes
(by @Not a tree)
f@0=0;f@i_:=i+f[i-1]
#Mathematica, 9 bytes
#(#+1)/2&
#Mathematica, 10 bytes
(#^2+#)/2&
#Mathematica, 11 bytes
Tr@Range@#&
#Mathematica, 12 bytes
i~Sum~{i,#}&
#Mathematica, 15 bytes Tr[#&~Array~#]&
#Mathematica, 16 bytes Binomial[#+1,2]&
#Mathematica, 18 bytes
PolygonalNumber@#&
#Mathematica, 19 bytes
#+#2&~Fold~Range@#&
#Mathematica, 9 bytes
#(#+1)/2&
#Mathematica, 10 bytes
(#^2+#)/2&
#Mathematica, 11 bytes
Tr@Range@#&
#Mathematica, 12 bytes
i~Sum~{i,#}&
#Mathematica, 15 bytes Tr[#&~Array~#]&
#Mathematica, 16 bytes Binomial[#+1,2]&
#Mathematica, 18 bytes
PolygonalNumber@#&
#Mathematica, 19 bytes
#+#2&~Fold~Range@#&
#Mathematica, 20 bytes
(by @Not a tree)
f@0=0;f@i_:=i+f[i-1]