Mathematics for IGCSE & O level - Pythagoras' Theorem And Trigonometry (Section 4 - No. 37)
What are the two conditions for using the Pythagorean Theorem?
Acute triangle; know the angle
Right triangle; know two sides
Obtuse triangle; know the hypotenuse
Isosceles triangle; know the angle
Explication
The Pythagorean theorem applies to right triangles, and you need to know the lengths of two sides to find the third.