已知向量a=(cosA,-2),b=(sinA,1)且向量a.b平行,则tan(A-圆周率/4)=?

问题描述:

已知向量a=(cosA,-2),b=(sinA,1)且向量a.b平行,则tan(A-圆周率/4)=?

cosa/-2=sina/1
-0.5=sina/cosa
tana=-0.5
tan(a-π/4)=(tana-tanπ/4)/(1+tana*tanπ/4)=-1.5/0.5=-3