这种题目需要用万能公式
和差化积：( 公式试卷上会给出）
sinx + siny = 2sin[(x+y)/2][cos[(x-y)/2] = 1/3
cosx+cosy = 2cos[(x+y)/2]cos[(x-y)/2)=1/4
两式相除 tan[(x+y)/2] = 4/3
tan(x+y) = 2 tan[(x+y)/2] /[1- tan[(x+y)/2]^2] = -24/7
sin2(x+y) = 2 tan(x+y)/[1+(tan(x+y))^2] = -336/625