Tricks on finding the sum of a sequence from 1 to a selected 1-digit number and back.

1. Choose a 1-digit number.
2. Square it.

Example:

Choose 6. Then, find the sum of 1+2+3+4+5+6+5+4+3+2+1. Instead of adding this manually, just simply take the square of 6.So, the sum is 36.

Thus, 1+2+3+4+5+6+5+4+3+2+1 = 36