Answer
$ x $
Work Step by Step
Suppose $\theta =\cos^{-1} (\dfrac{1}{x})$
This gives: $\cos \theta=\dfrac{1}{x}$
Since, $ r=\sqrt {x^2+y^2}$
We know $\sec \theta=\dfrac{1}{\cos \theta}=x $
Therefore, we have $\sec [\cos^{-1} (\dfrac{1}{x})]=x $
