$*A$ function $f(x)is$ defined $as$ follows:

$f(x)=\underset{tx}{lim}\frac{t-x}{sect-secx}$

Determine the value $f^{\text{'}}(\frac{}{4}).DO$ NOT use the definition $of$ the derivative $to$ solve this

problem, $i.e.do$ not use the fact that

$\underset{tx}{lim}\frac{sect-secx}{t-x}-=\frac{d}{dx}(secx)$