Hey guys, I have here

k(k+1)+2(k+1) / 2

which they have simplified to

(k+1)(k+2) / 2

Now I can see that they've multiplied that +1 in last term by the +2 constant which precedes that term, but why haven't they also made it 2k. Shouldn't it be (k+1)(2k+2)...? And how exactly was that leading k removed from it altogether...

Please help me out with this, I've been stuck for ages...