For any triangle with sides [i]a[/i], [i]b[/i], [i]c[/i], if [i]a[/i][sup]2[/sup] + [i]b[/i][sup]2[/sup] = [i]c[/i][sup]2[/sup], then the angle between [i]a[/i] and [i]b[/i] measures 90°.[br][br][code]LocusEquation[a^2+b^2==c^2,A][/code] asks GeoGebra where to put point [i]A[/i] in order to ensure [i]a[/i][sup]2[/sup] + [i]b[/i][sup]2[/sup] = [i]c[/i][sup]2[/sup].[br][br]Now, clearly, by putting [i]A[/i] on [i]d[/i], the angle between [i]a[/i] and [i]b[/i] is right.