tana/2
=sina/2 /cosa/2
=2sina/2cosa/2 / 2(cosa/2)^2
=sina/(1+cosa)
=(1-cosa)/sina
=1-(cosa)^2
=(1+cosa)(1-cosa)
三角函数两角和与差公式:
sin(A+B)=sinAcosB+cosAsinB
sin(A-B)=sinAcosB-cosAsinB
cos(A+B)=cosAcosB-sinAsinB
cos(A-B)=cosAcosB+sinAsinB
tan(A+B)=(tanA+tanB)/(1-tanAtanB)
tan(A-B)=(tanA-tanB)/(1+tanAtanB)