Converse of the Pythagorean theorem
For any triangle with sides a, b, c, if a2 + b2 = c2, then the angle between a and b measures 90°.
LocusEquation[a^2+b^2==c^2,A] asks GeoGebra where to put point A in order to ensure a2 + b2 = c2.
Now, clearly, by putting A on d, the angle between a and b is right.