Assume that P, O, and R are three points that are not all on one line. Assume that a, b, and c are scales. Let T be a point on the same plane with P, O, and R, and located inside the triangle PoR. Then we can write T = aP + bQ + cR for some scales a, b, and c. The condition a + b + c = 1 must be satisfied. What other conditions must a, b, and c satisfy?