If $\dfrac{a}{b} = \dfrac{3}{5}$, find the value of $\dfrac{a+b}{a-b}$.

By the componendo-dividendo identity, if $\dfrac{a}{b} = \dfrac{c}{d}$ then $\dfrac{a+b}{a-b} = \dfrac{c+d}{c-d}$. Plug in $c = 3, d = 5$: $\dfrac{3+5}{3-5} = \dfrac{8}{-2} = -4$. A common slip is to drop the sign and pick option C pay attention to $a < b$, which forces a negative denominator.