Something I remember from a math class

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What do all the numbers from 1 to 100 add up to and how do you figure it out without adding every single number?

Spoiler

The answer is 5050.

 

You can do this with ay string of numbers. You get it by adding the first and last number and multiplying it by half the number of numbers, So:

(1+100) x (100/2)-5050