# C4 vectors WJECWatch

#1
First post... scary stuff!

I have a past paper question to do for homework but I'm really not sure how to go about it. The question is...

The position vectors of the points A, B and C, relative to a fixed origin O, are 4i + 6j + k, 5i + 8j + 3k and 8i + 2j + 5k, respectively.
Find cos(ABC) [roof on B].

I would be extremely grateful for any help
0
7 years ago
#2
The points A, B and C form a triangle. So to find angle B use the cosine rule.
You'll need to work with the lengths AB, BC and AC.
0
7 years ago
#3
(Original post by Wales 1992)
First post... scary stuff!

I have a past paper question to do for homework but I'm really not sure how to go about it. The question is...

The position vectors of the points A, B and C, relative to a fixed origin O, are 4i + 6j + k, 5i + 8j + 3k and 8i + 2j + 5k, respectively.
Find cos(ABC) [roof on B].

I would be extremely grateful for any help
You need to use the dot product formula.
0
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