设A折叠后落在BD上的A'点,AG=X,A'G=AG=X,BG=AB-AG=2-X,BD²=AB²+AD²=AB²+BC²=2²+1²=5,BD=√5；∠GA'D=∠A=90°,A'D=AD=1,BA'=BD-A'D=√5-1,BG²-A'G²=BA'²(2-X)
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