P M P2 φ B -F x H AH B A 等代印KC F P2 B B HK  x y x y FAy FBy FAy 0 FBy 0 a2 b1 b2 a1 等代梁 FAH FBH l FP1 FP2 A B C l1 l2 f K FP1 FP2 A C B