The correct statement that identifies Hans' error is:
He should have added 9 + 9 to find the value of 9 squared.
This statement captures the essence of the mistake; Hans did not correctly compute the squares of the side lengths. Specifically, 9 squared should be calculated as \(9^2 = 81\), not just treated as the sum of 9 and 9. However, it's worth noting that the correct calculation for the hypotenuse in the Pythagorean theorem involves squaring the lengths and then adding them, which he incorrectly asserted as \(92 + 402\) (which should have been \(9^2 + 40^2\)).
If we are to highlight more precisely:
- The leg lengths should have been squared individually as \(9^2 = 81\) and \(40^2 = 1600\), yielding \(81 + 1600 = 1681\) for \(c^2\), leading to the hypotenuse \(c = \sqrt{1681}\), not 1618.
Therefore, there is ambiguity in some of the options you provided. The most directly related option regarding squaring is that he miscalculated the square of 9.