向量a=(cosa,-1),向量b=(2,sina),
若向量a垂直于向量b,
向量a*向量b=2cosa-sina=0
tana=2
tan(a-π/4)= (tana-tanπ/4) /(1+tana*tanπ/4)
=(2-1)/(1+2*1)=1/3