EXAMPLE $5$ Evaluate ${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}ta{n}^6(7x)se{c}^4(7x)dx$

SOLUTION If we separate one $se{c}^2(7x)$ factor, we can express the remaining $se{c}^2(7x)$ factor in terms of tangent using the identity $se{c}^2(7x)=1+ta{n}^2(7x).$ We can then evaluate the integral by substituting $u=tan(7x)$ so that $du=dx$