Like you rightly stated, in a right angled triangle. The phenomenon you mentioned is what us called Pythagoras theorem which proved to us that the square of the length of the hypotenuse of a right angled triangle, is equal to the square of the length of the opposite side to the angle in question, and also to the square of the length of the side adjacent to the same angle in question.
Essentially,
a² + b² = c², where c is the hypotenuse and a & b is the length of the two other sides
