连结OC、OD
首先,我用MN表示向量MN,注意此时字母MN有方向性
MD*NC
=(OD-OM)(OC-ON)
=OD*OC+OM*ON-OM*OC-OD*ON
=|OA|²*cos60°-|OM|²-|OM|*|OC|*cos120°-|OD|*|ON|*cos120°
=|OA|²/2-|OA|²/9+|OA|²/3
=18-4+12
=26
连结OC、OD
首先,我用MN表示向量MN,注意此时字母MN有方向性
MD*NC
=(OD-OM)(OC-ON)
=OD*OC+OM*ON-OM*OC-OD*ON
=|OA|²*cos60°-|OM|²-|OM|*|OC|*cos120°-|OD|*|ON|*cos120°
=|OA|²/2-|OA|²/9+|OA|²/3
=18-4+12
=26