Question

tg^2 9* - 1
-----------------
2tg9*

Answer 1

$frac{ {tg}^{2}9 - 1 }{2tg9} = frac{ - (1 - {tg}^{2}9) }{2tg9} = - frac{1}{ frac{2tg9}{1 - {tg}^{2}9 } } = - frac{1}{tg18}$