05AB1E, 7(削除) 7 (削除ここまで) 6 bytes
EDIT: Thanks to @Dennis: Saved a byte because I forgot about the increment operator
8*1+t8*>t.ï
n is triangular if sqrt(8n + 1) is an integer
How it works
8* # multiply implicit input by 8
1+> # add one
t # sqrt
.ï # is integer
05AB1E, 7 bytes
8*1+t.ï
n is triangular if sqrt(8n + 1) is an integer
How it works
8* # multiply implicit input by 8
1+ # add one
t # sqrt
.ï # is integer
05AB1E, (削除) 7 (削除ここまで) 6 bytes
EDIT: Thanks to @Dennis: Saved a byte because I forgot about the increment operator
8*>t.ï
n is triangular if sqrt(8n + 1) is an integer
How it works
8* # multiply implicit input by 8
> # add one
t # sqrt
.ï # is integer