题目内容：

cos2a/sin(a-pai/4)=-根号2／2求cosa＋sina

优质解答

cos2a/sin(a-pai/4)=-根号2／2
cos2a/(sinacos(pai/4)-cosasin(pai/4))=-根号2／2
cos2a/(根号2／2*sina-根号2／2*cosa)=-根号2／2
cos2a/根号2／2*(sina-cosa)=-根号2／2
(cos^2 a-sin^2 a)/(sina-cosa)=-1/2
(cosa-sina)(cosa+sina)/(sina-cosa)=-1/2
cosa+sina=1/2.