已知sin(3pai/4+a)=5/13,cos(pai/4-b)=3/5,且0

来源：学生作业学帮网编辑：学帮网时间：2024/05/17 08:18:37

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