Well, I think that question was relatively easy for this forum so I decided to share a slightly more challenging induction question:
prove for every positive integer n, if
$$a_{i} > 0 , b_{i}>0 $$ for all i = 1,..,n
then
\sum_{i=1}^{n}\frac{a_{i}b_{i}}{a_{i}+ b_{i}} \leq \frac{AB}{A+B}...