Chapter 6 - Section 6.7 - The Dot Product - Exercise Set - Page 794: 88

See the explanation below.

Consider the identity, $\begin{align} & {{\sin }^{2}}x{{\tan }^{2}}x+{{\cos }^{2}}x{{\tan }^{2}}x={{\sec }^{2}}x-1 \\ & {{\tan }^{2}}x\left( {{\sin }^{2}}x+{{\cos }^{2}}x \right)={{\tan }^{2}}x \\ & {{\tan }^{2}}x\left( 1 \right)={{\tan }^{2}}x \\ & {{\tan }^{2}}x={{\tan }^{2}}x \end{align}$