The following are lines from a proof by induction. Which line uses the inductive hypothesis?

Statement: For every positive integer n, 

Proof. 

1. Since 1=(1\cdot 2)/2, the statement is true for n=1.
2. Assume the statement is true for a positive integer k.
3. 1+2+\dots+k+(k+1) = k(k+1)/2 +(k+1)
4. Now k(k+1)/2+(k+1) = (k+1)(k/2 + 1) = (k+1)(k+2)/2
5. By the principle of mathematical induction, 1+2+\dots+n = n(n+1)/2 for every positive integer n.