Re: [線代] 一些內積的問題
※ 引述《mqazz1 (無法顯示)》之銘言:
: 1. evaluate u.v and ||u*v|| if ||u||=2, ||v||=5, ||u+v||=1, ||u-v||=5
Note that (u+v).(u+v) - (u-v).(u-v) = 4u.v. So
2 2
1 2 2 1 - 5
u.v = ── (∥u+v∥ -∥u-v∥ ) = ───── = -6.
4 4
u*v ??? What is it?? I don't understand......
: [0 0 -2]
: 2. A = [1 2 1], find the orthonormal basis for the column space of A
: [1 0 3]
: 為什麼書上先算eigenvalue = 1, 2
: 然後{[-2, 1, 1]}為1對應的eigenspace之一basis
: {[-1, 0, 1], [0, 1, 0]}為2對應的eigenspace之一basis
: 請問這個是題目要求的嗎?
我覺得直接用 Grad-Schmidt process 做就好了...... 可能他是想用
{[-2, 1, 1], [-1, 0, 1], [0, 1, 0]} 去做 Grad-Schidt process 吧!!
: 3. the area of a triangle with two sides given by a = <1, 3, -1> and
: b = <2, -1, 2> is ?
: [ 1 2]
: A=[ 3 -1]
: [-1 2]
: T
: 為什麼area = (1/2) * 根號[ det(A A) ]
: 謝謝
假設 a, b 兩根向量的夾角為 θ. 則
_______
1 1 ∕ 2
Area = ── ∥a∥∥b∥ sinθ = ── ∥a∥∥b∥ ∕ 1 - cos θ
2 2 ︹∕
_________________
1 ∕ 2 2 2 2 2
= ── ∕∥a∥ ∥b∥ - ∥a∥ ∥b∥ cos θ
2 ︹∕
____________
1 ∕ 2 2 2
= ── ∕∥a∥ ∥b∥ - (a.b)
2 ︹∕
___________
1 ∕ ┌ ∥a∥ a.b ┐
= ── ∕ det│ │
2 ︹∕ └ a.b ∥b∥ ┘
_____________
1 ∕ a T T
= ── ∕ det ([---][ a │ b ] )
2 ︹∕ b
______
1 ∕ T
= ── ∕ det (A A) .
2 ︹∕
冒昧問一下,你是大一剛在修線代的嗎? 還是在準備研究所??
因為之前看你問過很多線代的問題......
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