# Young Gauss’ Trick

Brain Hayes explores the meaning of one of most famous stories in mathematics.

Let me tell you a story, although it’s such a well-worn nugget of mathematical lore that you’ve probably heard it already:

In the 1780s a provincial German schoolmaster gave his class the tedious assignment of summing the first 100 integers. The teacher’s aim was to keep the kids quiet for half an hour, but one young pupil almost immediately produced an answer: 1 + 2 + 3 + … + 98 + 99 + 100 = 5,050. The smart aleck was Carl Friedrich Gauss, who would go on to join the short list of candidates for greatest mathematician ever. Gauss was not a calculating prodigy who added up all those numbers in his head. He had a deeper insight: If you “fold” the series of numbers in the middle and add them in pairs—1 + 100, 2 + 99, 3 + 98, and so on—all the pairs sum to 101. There are 50 such pairs, and so the grand total is simply 50×101. The more general formula, for a list of consecutive numbers from 1 through n, is n(n + 1)/2.

The paragraph above is my own rendition of this anecdote, written a few months ago for another project. I say it’s my own, and yet I make no claim of originality. The same tale has been told in much the same way by hundreds of others before me. I’ve been hearing about Gauss’s schoolboy triumph since I was a schoolboy myself.

The story was familiar, but until I wrote it out in my own words, I had never thought carefully about the events in that long-ago classroom. Now doubts and questions began to nag at me. For example: How did the teacher verify that Gauss’s answer was correct? If the schoolmaster already knew the formula for summing an arithmetic series, that would somewhat diminish the drama of the moment. If the teacher didn’t know, wouldn’t he be spending his interlude of peace and quiet doing the same mindless exercise as his pupils?

(Hat tip: Dan Balis)