第一个问题：∵ABC－A1B1C1是棱长为a的正三棱柱,∴BC＝A1C1、∠BCD＝∠A1C1D＝90°,又CD＝C1D,∴△BCD≌△A1C1D,∴BD＝A1D.∵ABC－A1B1C1是棱长为a的正三棱柱,∴ABB1A1是正方形,又A1B∩AB1＝G,∴A1G＝BG.由BD＝A1D、...