sin(3π+a)=2sin(3π/2+a),左边=sin(3π/2+3π/2+a)=sin(3π/2)cos(3π/2+a)+sin(3π/2+a)cos(3π/2)=-cos(3π/2+a)=cos(π/2+a)=-sina右边=2sin(3π/2+a)=-2sin(π/2+a)=-2cosa所以-sina=-2cosatana=2分母上是不是 ...