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by rainhacker 2488 days ago
I like 'Better Explained'[1]. It specifically focuses on intuitive understanding of mathematics.

A post I like is on adding numbers 1 to 100 [2]. The staple formula is n(n+1)/2, sum of arithmetic progression. How can we intuitively arrive that this formula?

> Technique 1: Pair Numbers Pairing numbers is a common approach to this problem. Instead of writing all the numbers in a single column, let’s wrap the numbers around, like this:

1 2 3 4 5

10 9 8 7 6

An interesting pattern emerges: the sum of each column is 11. As the top row increases, the bottom row decreases, so the sum stays the same.

Because 1 is paired with 10 (our n), we can say that each column has (n+1). And how many pairs do we have? Well, we have 2 equal rows, we must have n/2 pairs.

Number of Paris x Sum of Each Pair = (n/2) (n + 1) = n(n+1) / 2

[1] https://betterexplained.com/

[2] https://betterexplained.com/articles/techniques-for-adding-t...

1 comments

This reasoning for adding consecutive numbers is ok, but for some people the geometric reasoning (that you are adding half of a full square plus the diagonal) will be much clearer and intuitive. It also allows a visual generalization to higher dimensions, where the formula is not so evident.