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命题三棱锥P-ABC被任意平面所截,截得A1B1C1(如图1).则证明以△PA1B1及△PAB为底面,则
The propositional trigonal pyramid P-ABC is intercepted by an arbitrary plane and intercepted by A1B1C1 (Fig. 1). It is proved that ΔPA1B1 and △PAB are the bottom planes.