Dot Product

|A| : Magnitude of vector A

How to find it?
1. Usually given
2. If A is a unit vector (such as i,j), |A| = 1
3. If A = kU where U is a unit vector, |A| = k
4. Other geometric methods:
a) Pyth. theorem.
Let A = ai + bj
|A| = √(a^2 + b^2)
(Since vectors ai, bj and A form a right-angled triangle.)

b) Cosine Law
Let OB = b, OC = c,
Given |b| = 3, |c| = 4 and the angle between b and c is 60 degree, find |c - b|
|c - b| = √(|b|^2 + |c|^2 - 2|b| |c| cos 60 degree )
= √[3^2 + 4^2 - 2(3)(4)(1/2) ]
= √13

(|c - b| = Length of BC)

|A| 是 vector A 的 magnitude...
vector 是一個箭咀, 箭的長度是magnitude
箭咀的方向是direction