|a|=|b|=1a.b = 1/22a－b与b的夹角为xTo find : cosxSolution|2a-b|^2 = 4|a|^2+|b|^2 -4a.b =4+1-2 =3|2a-b| =√3|b|=1 |2a-b||b|cosx = (2a-b).b√3. cosx = 2a.b -|b|^2√3. cosx = 0cosx =0
