We are given that $\sec \theta + \tan \theta = k$.
We want to find the value of $\sec \theta - \tan \theta$.
We know the trigonometric identity: $$ \sec^2 \theta - \tan^2 \theta = 1 $$
We can factor the left side as a difference of squares: $$ (\sec \theta + \tan \theta) (\sec \theta - \tan \theta) = 1 $$ We are given that $\sec \theta + \tan \theta = k$. Substituting this into the equation, we get: $$ k (\sec \theta - \tan \theta) = 1 $$ Dividing both sides by $k$, we have: $$ \sec \theta - \tan \theta = \frac{1}{k} $$
Given that $\sin \theta + \cos \theta = x$, prove that $\sin^4 \theta + \cos^4 \theta = \dfrac{2 - (x^2 - 1)^2}{2}$.