Mathematics for IGCSE & O level - Pythagoras' Theorem And Trigonometry (Section 1 - No. 3)
What is the defining feature of a right-angled triangle that allows the direct use of Pythagoras' theorem?
It has three equal sides.
It has one angle that is 90 degrees.
It has two equal sides.
All angles in a triangle are equal
Explication
The presence of a 90-degree angle is essential for Pythagoras.