A$.$ State the best substitution to use when starting the following integral calculations, but do NOT integrate.

${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}\frac{x^3}{\sqrt[\phantom{2}]{1+x^2}}dx$

${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}si{n}^{5}xco{s}^{2}xdx$

${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}ta{n}^{3}xse{c}^{3}xdx$

${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}\frac{\sqrt[\phantom{2}]{x^2-4}}{x}dx$