sin(π-α)=-2sin(π/2+α),求sinα·cosα
问题描述：
sin(π-α)=-2sin(π/2+α),求sinα·cosα
答
化简sin(π-α)=-2sin(π/2+α),得sinα=-2cosα.
再联立(sinα)^2+(cosα)^2=1,求出(cosα)^2=1/5
sinα·cosα=-2(cosα)^2=-2/5