To prove that these numbers form the sides of a right triangle we should do these steps
1. Square the longest side
2. Find the sum of the squares of the other 2 sides
3. If the sum equals the square of the longest side, then the numbers can form a right triangle
Since the longest side is 41, then
Since the other 2 numbers are 40 and 9, then
From both steps, we can say
Then the answer is
YES, 9, 40, 41 can form the sides of a right triangle