Question

Rewrite 2tan112.5/1-tan^2(112.5) using a double-angle identity.

Answer 1

The following identity can be used:
$tan\ 2\theta=\frac{2\ tan\theta}{1-tan^{2}\theta}$
$\frac{2\ tan(112.5)}{1-tan^{2}(112.5)}=tan(2\times112.5)=tan(225)$