(x p /x q ) p + q . (x q /x r ) q + r . (x r /x p ) r + p = (x p - q ) p + q . (x q - r ) q + r . (x r - p ) r + p = x (p - q)(p + q) . x (q - r)(q + r) . x (r - p)(r + p) = x p 2 - q 2 . x q 2 - r 2 . x r 2 - p 2 = x p 2 - q 2 + q 2 - r 2 +r 2 - p 2 = x 0 = 1
