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Can you find a quick and elegant way to add the numbers from 1 to 28 ?

In other words, what is the sum you obtain in the following case:

1 + 2 + 3 + ....... + 28 =
Answer: 406

Solution:

Here's an elegant way:

1 + 28 = 29
2 + 27 = 29
3 + 26 = 29
.....................
.....................
14 + 15 = 29

Adding the above equations gives

1 + 2 + 3 + ....... + 28 = 14 × 29 = 406

Food for thought:

The general formula is:

1 + 2 + 3 + ....... + n = n (n + 1) / 2.

Is the formula valid when n is an odd integer?


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puzzle : image for addition

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