Question

If `A = <-7 ,1 ,5 >`, `B = <3 ,6 ,-2 >` and `C=A-B`, what is the angle between A and C?

Answer 1

The angle is `=28.9`º

Explanation:

Let's start by calculating

`vecC=vecA-vecB`

`vecC=〈-7,1,5〉-〈3,6,-2〉=〈-10,-5,7〉`

The angle between `vecA` and `vecC` is given by the dot product definition.

`vecA.vecC=∥vecA∥*∥vecC∥costheta`

Where `theta` is the angle between `vecA` and `vecC`

The dot product is

`vecA.vecC=〈-7,1,5〉.〈-10,-5,7〉=70-5+35=100`

The modulus of `vecA`= `∥〈-7,1,5〉∥=sqrt(49+1+25)=sqrt75`

The modulus of `vecC`= `∥〈-10,-5,7〉∥=sqrt(100+25+49)=sqrt174`

So,

`costheta=(vecA.vecC)/(∥vecA∥*∥vecC∥)=100/(sqrt75*sqrt174)=0.88`

`theta=28.9`º