|a+tb|^2 ＝(a+tb)*(a+tb) ＝|b|^2t^2+2(a*b)t+|a|^2 ＝|b|^2t^2+2|a|*|b|cosθ t+|a|^2 ＝|b|^2[t^2+2|a|cosθ/|b| t]+|a|^2 ＝|b|^2[t+|a|cosθ/|b|]^2+|a|^2(sinθ)^2.最小值在t＝－|a|cosθ/|b|处取得.此时,b*(a...