The correct statement that identifies Hans's error is: He did not square 40, he just multiplied by 2.
In the Pythagorean theorem, the correct method to find the length of the hypotenuse \(c\) is to square both legs of the triangle and then add them together. It seems that Hans made an error in calculating \(40^2\) and instead inaccurately performed a different operation.
For a right triangle with legs of lengths 9 and 40, the correct formula should be:
\[ 9^2 + 40^2 = c^2 \]
Calculating that gives:
\[ 81 + 1600 = c^2 \] \[ 1681 = c^2 \] \[ c = \sqrt{1681} = 41 \text{ cm} \]
So, the process that should have been followed involves squaring the lengths, not just multiplying.