DOT PRODUCT.

When performing scalar operations(dot product) with vectors ie (a*b*cos0)

Is it that you use x*x*cos0? or x*y*cos0

As an example cos = 66

x2-66 y2-148
x1-73 y1-34


Would it be

1)x2*x1*cos66? or

2)y2*x1*cos66 or

3)x2*y1*cos66 or

4)y2*y1*cos66?

What is the pattern or sequence? is it a set sequence? or it depends on the direction of the vector i.e up or down?

I am looking for educative tips that would prove helpful.
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