6. (2023春•镇巴县期末)已知锐角α\alphaα满足tan2α=43\tan 2\alpha =\frac{4}{3}tan2α=34,则sin2α−3cos(α+π2)cosα=({\sin ^2}\alpha -3\cos ({\alpha +\frac{\pi }{2}})\cos \alpha =(sin2α−3cos(α+2π)cosα=( )))