cos(a+b)=3/5 sin(b-pai/4)=5/13 a,b 属于(0,pai/2) 求cos(a+pai/4)

cos(a+b)=3/5 sin(b-pai/4)=5/13 a,b 属于(0,pai/2)
sin(a+b)=4/5 cos(b-pai/4)=12/13
cos(a+pai/4)=cos[(a+b)-(b-pai/4)]
=cos(a+b)cos(b-pai/4)+sin(a+b)sin(b-pai/4)
=3/5*12/13+4/5*5/13
=56/65