Answer
See below.
Work Step by Step
Proofs using mathematical induction consist of two steps:
1) The base case: here we prove that the statement holds for the first natural number.
2) The inductive step: assume the statement is true for some arbitrary natural number $n$ larger than the first natural number; then we prove that then the statement also holds for $n + 1$.
Hence, here we have:
1) For $n=1: 2=1(1+1)$.
2) Assume for $n=k: 2+4++2k=k(k+1)$. Then for $n=k+1$:
$2+4++2k+2(k+1)=k(k+1)+2(k+1)=k^2+k+2k+2=(k+1)(k+2)=(k+1)(k+1+1).$
Thus we proved what we wanted to.