3 ms·
no spaces. but you can get the formula by imagining the numbers as piles of pennies and then completing them to make a square and then splitting down the diago
by andrewcooke 13y ago
no spaces.
but you can get the formula by imagining the numbers as piles of pennies and then completing them to make a square and then splitting down the diagonal
*
**
***
*..
**.
***
\.. \
*\. + \
**\ \
where a \ is half a penny. so * = \\
so it's 1/2 * n * n (ie half the square) plus an extra 1/2 for each one on the diagonal.
so it's n * n/2 + n/2 = (n+1) * n/2