>>394
D = a1b2 - a2b1 + b1c2 - b2c1 + c1a2 - c2a1
0 < p1(a2 - b2) - p2(a1 - b1) + a1b2 - a2b1 < D
0 < p1(b2 - c2) - p2(b1 - c1) + b1c2 - b2c1 < D
0 < p1(c2 - a2) - p2(c1 - a1) + c1a2 - c2a1 < D