Proof. We have seen here that:
\begin{align*} \sin^2(x) + \cos^2(x) = 1. \end{align*}
\begin{align*} \frac{\sin^2(x)}{\cos^2(x)} + 1 = \frac{1}{\cos^2(x)}. \end{align*}
\begin{align*} \tan^2(x) + 1 = \sec^2(x). \end{align*}
\begin{align*} \sin^2(x) + \cos^2(x) = 1. \end{align*}
\begin{align*} \frac{\sin^2(x)}{\cos^2(x)} + 1 = \frac{1}{\cos^2(x)}. \end{align*}
\begin{align*} \tan^2(x) + 1 = \sec^2(x). \end{align*}