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# Angle between 2D vectors? Watch

1. Hi all,

I'm currently doing vectors in my Computer Science course.
I'm stuck on a question and thought I would try here for help.

Ive been given the vectors
A = (1,4)
B = (-2,4)

I need to work out the angle between the two.

I thought that I had to find the dot product then do cos on the dot product.

I think A . B = 14?

Help i'm confused.
2. (Original post by nathan201291)
Hi all,

I'm currently doing vectors in my Computer Science course.
I'm stuck on a question and thought I would try here for help.

Ive been given the vectors
A = (1,4)
B = (-2,4)

I need to work out the angle between the two.

I thought that I had to find the dot product then do cos on the dot product.

I think A . B = 14?

Help i'm confused.
Yeah you're on the right lines, you need to use the two properties of the dot product.
Firstly A.B=|A||B|cos(x)
also A.B=(1*-2+4*4)
You get |A||B|cos(x)=14
|A|=sqrt(1+16)
|B|=sqrt(4+16)
so cos(x)=14/(sqrt(17*20))
then solve for x

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