This demonstrates that [b]if[/b] the square of the longest side is equal to the sum of the squares of the other two sides then the angle opposite the longest side [b]must be 90 degrees.[/b]

Drag the slider to change the size of the angle A.[br][br]Notice that [b]only[/b] when Pythagoras' theorem applies is the angle opposite the longest side [b]90 degrees.[/b]