[tex]\displaystyle\int\tan^6\theta\sec^2\theta\,\mathrm d\theta[/tex]
[tex]x=\tan\theta\implies\mathrm dx=\sec^2\theta\,\mathrm d\theta[/tex]
[tex]\displaystyle\int x^6\,\mathrm dx=\dfrac{x^7}7+C[/tex]
[tex]\implies\displaystyle\int\tan^6\theta\sec^2\theta\,\mathrm d\theta=\dfrac{\tan^7\theta}7+C[/tex]