∵,|AB|=|AD|=2,|BD|=2√3, BD=AD-AB∴|AD-AB|=|BD|=2√3两边平方: |AD|²+|BD|²-2AD●AB=12∴4+4-2AD●AB=12∴AD●AB=-2∴cos
=AD●AB/(|AD||AB|)=-1/2∴∠DAB=120º∵向量AD=1/2向量BC∴AD//BC∴∠ABC=60º∴数量积BC●BA=|BC||BA|cosB=4*2*(1/2)=4
∵,|AB|=|AD|=2,|BD|=2√3, BD=AD-AB∴|AD-AB|=|BD|=2√3两边平方: |AD|²+|BD|²-2AD●AB=12∴4+4-2AD●AB=12∴AD●AB=-2∴cos
=AD●AB/(|AD||AB|)=-1/2∴∠DAB=120º∵向量AD=1/2向量BC∴AD//BC∴∠ABC=60º∴数量积BC●BA=|BC||BA|cosB=4*2*(1/2)=4