<p>If <img alt="$f(n)=\tfrac{1}{3} n(n+1)(n+2)$" height="21" src="https://latex.artofproblemsolving.com/7/c/d/7cd8367e6fd8a2ed43cc1b2105a2ba30125b3c08.png" width="192" />, then <img alt="$f(r)-f(r-1)$" height="18" src="https://latex.artofproblemsolving.com/1/c/3/1c3c5694bc547b9e68eb38ca8eaae74269c2d398.png" width="119" /> equals:</p> <p> </p>