有简单点的吗

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易证BO=5.
由正弦定理得到
BO/sin∠OCB=BC/sin120
sin∠OCB=5根号3/14
cos∠OCB=根号[1-(sin∠OCB)^2]=11/14
sin∠OBC=sin(180-120-∠OCB)=sin(60-∠OCB)=sin60cos∠OCB-cos60sin∠OCB=3根号3/14
S四边形ABCD=2S△BCD=BC*BD*sin∠OCB=15根号3