本文介绍八个三角函数的定义
（sin 正弦 , cos余弦 , tan正切 , cot余切 , sec正割 , csc余割 , vers正矢 , covers余矢）
如图所示，单位圆的半径为1。
则有 :
sin α = AB/OA = AB
cos α = OB/OA = OB
tan α = AB/OB = TC/OC = TC
cot α = OB/AB = 1/tan α = ES/EO = ES ，
sec α = 1/cos α = OA/OB = OT/OC = OT ,
csc α = 1/sin α = OA/AB = OS / OE = OS ,
vers α = 1 - cos α = 1 - OB = BC ,
covers α = 1 - sin α = 1 - AB = ED
