Question

Who invented this rule - sum of the squares of the legs of a triangle is equal to the square of the hypotenuse

Answer 1

It was invented by the great mathematician Pythagoras 

It is known as Pythagoras Theorem .Which says- that sum of the squares of the legs of triangle is equal to the square of the hypotenuseIn shot:- Hypotenuse ²= Base²+Length²

Answer 2

The theorem is named after the ancient Greek mathematician Pythagoras.
Hence, it is called as pythagoras theorem which states that,
sum of the squares of the legs of a triangle is equal to the square of the hypotenuse.
But, also Indian mathematicians and Chinese mathematicians founded it independently, without knowing that of the pythagoras.