4. Выполните действие: а) \(\dfrac{y-5}{3y}+\dfrac{y+15}{9y}\); б) \(\dfrac{1}{m-6}-\dfrac{1}{m+6}\).

\(\dfrac{y-5}{3y}+\dfrac{y+15}{9y}=\dfrac{3(y-5)+(y+15)}{9y}=\dfrac{4y}{9y}=\dfrac49\), при \(y\ne0\).

\(\dfrac{1}{m-6}-\dfrac{1}{m+6}=\dfrac{(m+6)-(m-6)}{(m-6)(m+6)}=\dfrac{12}{m^2-36}\), при \(m\ne6\) и \(m\ne-6\).