由tan(π+α)=1/2,可得tanα=1/2所以sin(α-7π)cos(α+5π)=-sinα * (-cosα)= sinα cosα= 1/2sin2α=1/2 * 2tanα/[1+tan2(α)]=tanα/[1+tan2(α)]=1/2 /(1+1/4)=2/5
5π/6
