a+b∈(7/4π,2π)
所以sin(a+b)0
sin²(a-b)+cos²(a-b)=1
所以sin(a-b)=3/5
cos2a
=cos[(a+b)+(a-b)]
=cos(a+b)cos(a-b)-sin(a+b)sin(a-b)
=-7/25