The known formula for the sum of the first n natural numbers is not intuitive at all. One proof for that formula is to duplicate the numbers and arrange it in pairs which sums up to and then sum up all the numbers:
It is a really nice proof and also very direct and intuitive. But I have come up with this alternative:
Suppose there are people meeting and they all shake hands with each other. To count how many hand shakes are in total, one can start counting the hand shakes for one person which is n. Then for a second person count her hand shakes, but don't count the hand shake with the first person, because that one was already counted, so in total, for the second person, we count , and so on. The total hand shakes is then the sum of the first n natural numbers.
But now we can count the hand shakes in a simpler way. There are people in total, and each of them shakes hand with n people. Then is twice the hands shake, because we are counting twice each hand shake. So the total hand shakes must be
The two ways of counting have to arrive to the same number so the formula holds.
I wonder how can this proof be formalized so it would be accepted as a real mathematical proof.