Which of these sets of three numbers can be the side lengths of a right-angled triangle?

Explanation

1. 01

   Check whether the squares of the two smaller numbers add up to the square of the largest.

2. 02

   $20^2 + 21^2 = 400 + 441 = 841 = 29^2$, so 20, 21, 29 works.

3. 03

   The others miss by one: $9^2 + 40^2 = 41^2$, $11^2 + 60^2 = 61^2$ and $12^2 + 16^2 = 20^2$, so 42, 62 and 21 are wrong.