That's a good question and very conceptual. Try to break your problem into a general summation which can be expressed by a variable.
Don't you worry about n(0), that thing is created to be zero, as you'll see in next step.
We can take the numbers outside the brackets as r, whose value varies from r=1,2,...,(n-1),n.
Now about the summation of numbers inside the bracket. You can take out their sum by AP. Since the first term of any bracket is (r+1) and the last term is always n, the number of terms can be figured as (n-r), since you're not counting first r natural number.
Therefore, our general term can be written as:
NOTE: Try considering r=n. It is a huge relief that T
n=0, or else we would have to manually subtract it in the end. Always consider manually checking last term in the general term, they may give you a problem.
Now fearlessly take summation of our series.
Now open it and you'll get the following:
Simple equation in summation of r, r
2 and r
3