k不等于0(a+kb).(ka+b)=k|a|^2 + k|b|^2 + (1+k^2)|a||b|cos120度= k + 2k-(1+k^2)= -k^2+3k-1 >0k^2-3k+1<0(3-√5)/2a+kb与ka+b的夹角为锐角(3-√5)/2
