cos2a=cosa^2-sina^2=(cosa+sina)*(cosa-sina)
sin(a-π/4）=sina*cos(π/4)-cosa*sin(π/4)=√2(sina-cosa)/2
cos2a 除以 sin(a-pai/4) ＝ 负的2分支根号2
即：(cosa+sina)*(cosa-sina)/[√2(sina-cosa)/2]=-√2/2
故cosa+sina=1/2
选C