Question

∑n(n+2) = ?
Ma ajutati si pe mine ?

Answer 1

∑n·(n+2)=∑n² +∑2n=∑n² +2·∑n=n·(n+1)·(2n+1) /6 +2n·(n+1) /2=n·(n+1)·(2n+1) /6 +n·(n+1)=n·(n+1)·(2n+1)+6n·(n+1) /6=n·(n+1)·(2n+7) /6 =>

∑n·(n+2)=n·(n+1)·(2n+7) /6 .