Integrate using trigonometric substitution

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

$(b){\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}\frac{x}{\sqrt[\phantom{2}]{x^2-9}}dx$