(2sinα+cosα)/(sinα-3cosα)=-5 求cos2α
问题描述：
(2sinα+cosα)/(sinα-3cosα)=-5 求cos2α
答
2sinα+cosα=-5sinα+15cosα
7sinα=14cosα
tanα=0.5
cos2α=((cosα)^2-(sinα)^2)/((cosα)^2+(sinα)^2)
=(1-(tanα)^2)/(1+(tanα)^2)
=3/5