For any four points P, Q, R, S, ∣∣→PQ×→RS−→QR×→PS+→RP×→QS∣∣ is equal to 4 times the area of the triangle.
A rectangular hyperbola whose centre is C, is cut by any circle of radius r in four points P, Q, R and S. Then, CP2+CQ2+CR2+CS2 is equal to
Through any given set of four distinct points P, Q, R and S, it is possible to draw at most ___circle(s).