Here's another challenge problem in case anyone would like a try besides the mundane.

Find the sum of all positive integers n for which is a perfect square.

Quote:

---

Originally Posted by galactus

Here's another challenge problem in case anyone would like a try besides the mundane.

Find the sum of all positive integers n for which is a perfect square.

---

Using Microsoft Excel shows that the expression equaled a perfect square in the following sequence for n:

2 + 21 = 23 + 23 = 46 + 25 = 71 + 27 = 98 + 29 = 127... etc

This shows that the increment between integers has a definite pattern, i.e. each new increment is 2 more than the previous increment.

Based upon this observation, the sum of the integers resulting in the target formula equaling a perfect square would be infinite.

I am sorry, I am not following. But, I don't think there are infinite solutions. Here's how I done it.

Let

Then we have the quadratic equation

By the quadratic formula, we get

Since n is an integer, must be a square, say it is

Then we have

We have two squares that differ by 35.

The only such squares are

That is

If

If

The sum of the n's is

I made a drastic error when constructing my spreadsheet.

Here is the corrected spreadsheet results which validates your answer.

N N^2 -19N + 99 RESULT SQRT (RESULT)

1 1.000000 80.000000 81.000000 9.00
9 81.000000 -72.000000 9.000000 3.00
10 100.000000 -91.000000 9.000000 3.00
18 324.000000 -243.000000 81.000000 9.00