let x =a,b 的夹角
m= a+tb
|m|^2 = |a|^2 + |b|^2 + 2t|a||b|cosx
t =1/4
|m|^2 = 1+4+cosx = 5 + cosx
min |m|^2 at cosx =-1
min |m| = 2
a,b 的夹角 = 180°