(-sina)(-cosb)+(-cosa)(-sinb)=-1/2 即sinacosb+cosasinb=-1/2 sin(a+b)=-1/2,a+b=2kπ-π/6或2kπ-5π/6,k∈Z 又a∈(π/2,π),b∈(π,2π) 则a+b∈(3π/2,3π) 即a+b=2π-π/6 =11π/6