Evaluate:

- \(\sum_{r=1}^n \dfrac{1}{r(r+1)(r+3)}\).

How can we rewrite \(\dfrac{1}{r(r+1)(r+3)}\) in a way that might be useful? Could we use partial fractions?

Or alternatively, if we write the numerator as \(r+1-r\), can we now split the fraction into two fractions which both have terms that cancel?