sin(a+b)=-3/5,sin(b-派/4)=12/13,ab属于（3派/4,派）求cos(a+派/4)_数学解答

sin(a+b)=-3/5,sin(b-派/4)=12/13,ab属于（3派/4,派）
求cos(a+派/4)

人气：421 ℃　时间：2020-06-02 20:29:04

解答

cos(a+pi/4) = cos(a+b-b+pi/4) = cos(a+b)cos(b-pi/4)+sin(a+b)sin(b-pi/4) = -4/5*5/13-3/5*12/13 =-56/65