Question 21
If $$
f\left(\frac{x}{x-1}\right)=\frac1x
$$ for all $x\notin\{0,1\},$ and $0\lt\theta\lt\frac{\pi}{2},$ then $f(\sec^2\theta)=$
f\left(\frac{x}{x-1}\right)=\frac1x
$$ for all $x\notin\{0,1\},$ and $0\lt\theta\lt\frac{\pi}{2},$ then $f(\sec^2\theta)=$