|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|a-λb|^2==|a|^2+λ^2|b|^2-2λa.b我算出来是这样的不好意思，弄错！|a-λb|^2=|a|^2+λ^2|b|^2-2λa.b= 1+λ^2+λ= (λ+ 1/2)^2 + 3/4min |a-λb|^2 at λ = -1/2min |a-λb| = √3/2