解ï¼åå«åCC1ãA1C1çä¸ç¹EãFï¼è¿ç»EFãBEãBFãB1F.
âµç´ä¸æ£±æ±ABC-A1B1C1
â´BB1//CC1ä¸BB1=CC1
âµDãEåå«æ¯BB1ãCC1çä¸ç¹
â´BD=(1/2)BB1ï¼C1E=(1/2)CC1
â´BD//C1Eä¸BD=C1E.
â´å边形BDC1Eæ¯å¹³è¡å边形.
â´BE//C1D
âµEãFåå«æ¯CC1ãA1C1çä¸ç¹
â´EFæ¯â³A1C1Cçä¸ä½çº¿
â´EF//A1C
â´â BEFå°±æ¯æ±å¼é¢ç´çº¿A1Cä¸C1Dææçè§.
ç±é¢æï¼B1F^2=A1B1^2+A1F^2=a^2+(a/2)^2=5a^2/4.
åBF^2=BB1^2+B1F^2=(2a)^2+5a^2/4=21a^2/4.
èBE=â(BC^2+CE^2)=â(AB^2+AC^2+CE^2)=â(a^2+a^2+a^2)=(â3)a
EF=â(C1E^2+C1F^2)=â[a^2+(a/2)^2]=(â5)a/2
â´cosâ BEF=(BE^2+EF^2-BF^2)/(2*BE*EF)=-(â15)/15
â´å¼é¢ç´çº¿A1Cä¸C1Dææè§ç大å°å°±æ¯arccos[-(â15)/15].
温馨提示:答案为网友推荐,仅供参考