hmmm...so i need to find an expression for the attached image...
im getting tan((x^2-4)/4) but its wrong, what am I missing?
Thanks in advance
Let $\displaystyle \alpha = \cos^{-1} \left( \frac{2}{x} \right) \Rightarrow \cos \alpha = \frac{2}{x}$. So you want the value of $\displaystyle \tan^2 \alpha$.
Now note that $\displaystyle \tan^2 \alpha = \frac{1}{\cos^2 \alpha} - 1$. Make the necessary substitution and simplify.