Sum of first N natural numbers

The sum of first N Natural numbers is .... N * (N+1) / 2 Prove it. So let's assume Sum as Sum = 1 + 2 + 3 + ......N-2+N-1+N Now Reverse it. Sum = N+N-1+N-2+.....3+2+1 Add these. Sum = 1 + 2 + 3 + ......N-2+N-1+N Sum = N+N-1+N-2+.....3+2+1 2Sum = (N+...