Which equation is used in the proof of the Four-Square Theorem?
x2+y2+z2+w2=nx^2 + y^2 + z^2 + w^2 = nx2+y2+z2+w2=n
x2+y2=nx^2 + y^2 = nx2+y2=n
x3+y3=z3x^3 + y^3 = z^3x3+y3=z3
y2=x3+xy^2 = x^3 + xy2=x3+x