3. （2023春•岳麓区校级月考）若cosα−sinα=−12\cos \alpha -\sin \alpha =-\frac{1}{2}cosα−sinα=−21，则sinαcosαtan2(α−π4)=(\frac{\sin \alpha \cos \alpha }{ta{n}^{2}(\alpha -\frac{\pi }{4})}=(tan2(α−4π)sinαcosα=( )))