已知向量a=(sinx,3/4),b=(cosx,-1).当a‖b时求cos^2x-sin^2x的值
问题描述：
已知向量a=(sinx,3/4),b=(cosx,-1).当a‖b时求cos^2x-sin^2x的值
答
a//b
=> sinx/(3/4) = cosx/(-1)
-4sinx=3cosx
tanx = -3/4
(cosx)^2 = (4/5)^2 = 16/25
(cosx)^2 - (sinx)^2
= 2(cosx)^2-1
=2(16/25)-1
=7/25