|a|=2,|b|=1.a.b = |a||b|cos丌/3a.b = 1|2a+3b|^2 =(2a+3b).(2a+3b)=4|a|^2+9|b|^2+ 12a.b=16+9+12=37|3a-b|^2 = 9|a|^2+|b|^2 - 6a.b =36+1-6=31let 2a+3b,与3a-b的夹角 = x(2a+3b).(3a-b)= |2a+3b||3a-b|cosx6|a|...