x垂直于y
=> x.y =0
=>(a+(t^2-3)b).(-ka+tb) =0
-k|a|^2+t(t^2-3)|b|^2+(t-k(t^2-3))a.b =0
-4k + t(t^2-3) + (t-k(t^2-3))( 2cos(-A)cos(90°-A) + 2sin(-A)sin(90°-A) =0
-4k + t(t^2-3) =0
k = t(t^2-3)/4
let
S = (k+t^2)/t
=( t(t^2-3)/4 + t^2)/t
= ( t^2+4t-3) /4
S' = (2t+4)/4 =0
t = -2
S'' = 1/2 >0 ( min)
minS at t=-2
min S = ( 4-8 -3) /4 = -7/4o(∩_∩)o 感谢！！~这些知识都搞不清了，顺便问一下，俩个复杂的向量相乘是不是能用多项式相乘的方法？