$\rm tan^{-1}x+cot^{-1}x=\frac{\pi}{2}$ holds, when

If $\tan A=\dfrac{1}{7}$, then what is cos 2A equal to?

The sides of a triangle are m, n and $\rm \sqrt{m^2+n^2+mn}$. What is the sum of the acute angles of the triangle?

What is the area of the triangle ABC with sides a = 10cm  and c = 4cm angle B = 30°?