The legs of a right triangle are 12 and 5. Find the hypotenuse.

1) In order to find the hypotenuse, when two legs of a right-angled triangle are known, you need to use the Pythagorean theorem: the sum of the squares of a leg is equal to the square of the hypotenuse.
2) Let us denote the hypotenuse of the triangle by X.
3) Let’s compose the equations following the theorem:
12 ^ 2 + 5 ^ 2 = X ^ 2;
144 + 25 = X ^ 2;
169 = X ^ 2;
X = 13 (√169).
Answer: 13.