The first line = (1+2) 2 = 3 2

The second line = (1+2+3) 2 = 6 2

The third line = (1+2+3+4) 2 = 10 2.

The fourth line = (1+2+3+4+5) 2 = 212.

So the last line should be = (1+2+3+...+n) 2.

Prove by mathematical induction