正弦定理:2/sin∠ADB=BD/sin30°=2BD,1/sin∠ADC=CD/sin∠DAC
依题意:BD=CD,且有sin∠ADB=sin∠ADC
所以sin∠DAC=BD*sin∠ADB=2*sin30°=1,则∠DAC=90°,所以∠BAC=120°
余弦定理:BC^2=AB^2+AC^2-2AB*AC*cos∠BAC=4+1+4*1/2=7
所以BC=7^0.5
正弦定理:2/sin∠ADB=BD/sin30°=2BD,1/sin∠ADC=CD/sin∠DAC
依题意:BD=CD,且有sin∠ADB=sin∠ADC
所以sin∠DAC=BD*sin∠ADB=2*sin30°=1,则∠DAC=90°,所以∠BAC=120°
余弦定理:BC^2=AB^2+AC^2-2AB*AC*cos∠BAC=4+1+4*1/2=7
所以BC=7^0.5