tangent n. 切線；正切值
tangent line 切線
secant n. 割線
secant line 割線
an auxiliary variable 輔助變量
Induction 歸納法
Deduction 演繹法
trig / trigonometric identities 三角恆等式
( omitting length: )
sine = 1 / cosecant = the opposite side / the hypotenus
cosine = 1 / secant = the adjacent side / the hypotenuse
tangent = 1 / cotangent = the opposite side / the adjacent side