|a|=|b|=1
a,b夹角=120°
|a-λb|^2
=(a-λb).(a-λb)
=|a|^2+|b|^2-2λa.b
=2(1-λcos120°)
|a-λb|(λ属于实数)取最小值时
1-λcos120° =0
λ = -2