bb16
幼苗
共回答了20个问题采纳率:85% 举报
因为AB=AC=√5,BC=2所以,cosCAB=(AB²+AC²-BC²)/(2AC*AB)=3/5所以,sinCAB=4/5因为,AB平行于CD,所以ACD=BAC所以,sinDAC=sin(180-2*ACD)=sin(2*ACD)=sin(2*BAC)=2*sinBAC*cosBAC=24/25所以,cosDAC=7/25所以,cosDAB=cos(DAC+BAC)=cosDAC*cosBAC-sinDAC*sinBAC=-3/5而AD=AB=√5所以BD=√[AD²+AB²-2AD*AB*cosDAB]=√[16]=4
1年前
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