What is the Pythagoreum Theorem?

Pythagoras theorem is relates the sides of a
triangle.

In a right angled triangle the sum of the squares
of the sides that contain the right angle is equal to the square on the the third
side.

Let ABC be a right angled triangle with right angle
at B. Then AB ,  BC  are the sides that contain right angle . Then Ac is the side
opposite to  the right angle. Then  AC^2 = AB^2+BC^2.

It is
also true that in any triangle ABC if sum of the squares of any two sides AB and BC  is
equal to the third side AC, then the triangle is a right angled triangle with right
angle contained by AB and BC.

Pythagoras theorem could be
used to  verify  whether a triangle is right angle. It could be used to create 90 degree
measure of angle with ordinary scale (not even calibrated) .