即使用tan(a/2)表示角a的三角函数(其中tan^2(a/2)=tan(a/2)*tan(a/2))sina=(2tan(a/2))/(1+tan^2(a/2))cosa=(1-tan^2(a/2))/(1+tan^2(a/2))tana=(2tan(a/2))/(1-tan^2(a/2))cota=(1-tan^2(a/2))/(2tan(a/2))seca=(1+tan^2(a/2))/(1-tan^2(a/2))csca=(1+tan^2(a/2))/(2tan(a/2))