已知tan(派-a)=-2,求3sina+2cosa5sina-cosa 求sina•cosa

题目

已知tan(派-a)=-2,求3sina+2cosa5sina-cosa 求sina•cosa

答案

tan(派-a)=-2 => tana=2
(3sina+2cosa)/(5sina-cosa)
=(3tana+2)/(5tana-1)
=8/9
sina•cosa
=(1/2)sin(2a)
=tana/(tan^2a+1)
=2/5