|a+λb|²=(a+λb).(a+λb)=a.a+a.λb+λb.a+λb.λb=λ²(b.b)+2λ(a.b)+a.a=(b.b)(λ²+2λ(a.b)/b.b+a.a/b.b)=(b.b)((λ+(a.b)/(b.b))²+a.a/b.b-(a.a/b.b)²)b.b≥0,当 λ+(a.b)/(b.b)=0时 ...