Pythagoras' theorem states that for all right-angled triangles, 'The square on the hypotenuse is equal to the sum of the squares on the other two sides'. The hypotenuse is the longest side and it's always opposite the right angle.
In this triangle and angle is a right angle.
Pythagoras' theorem only works for right-angled triangles, so you can use it to test whether a triangle has a right angle or not.
In the triangle above, if the angle is acute.
In the triangle above, if the angle is obtuse.
Which of the following triangles is right-angled?
is greater than ,
so angle must be obtuse.
is equal to
so angle must be a right angle.
is less than
so angle must be acute.
So, it is triangle b which is right-angled.