因为(3π/4+B)-(π/4-a)=B+a+π/2
所以cos[（a+B）+π/2]=cos(a+B)*cosπ/2-sin(a+B)*sinπ/2=-sin(a+b)
所以sin(a+b)=-cos[（a+B）+π/2]=-cos[(3π/4+B)-(π/4-B)]
=-[cos(3π/4+B)*cos(π/4-a)+sin(3π/4+B)*sin(π/4-a)]
由π/4