If $\displaystyle tan A=\ sec2\alpha\ cot\pi$ prove that $\displaystyle \frac{cos(A+\pi)}{cos(A-\pi)}=-tan^2 \alpha$ I tried this by simplifying the statement and finally i get $\displaystyle \frac{cos(A - \pi)}{cosAcos\pi}=\frac{2cos^2\alpha}{2cos^2\alpha-1}$ then i can't simplify this prove.please help me!!!