$(9, 40, ?)$ is a Pythagorean triple. The third member is

$(9, 40, ?)$ is a Pythagorean triple. The third member is
  1. $41$
  2. $49$
  3. $39$
  4. $45$

Solution

$9^{2} + 40^{2} = 81 + 1600 = 1681 = 41^{2}$.

Asked in: IMO

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