(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

Prove that A*B = |A||B|cos theta

2. Relevant equations

Law of cosines: c^2= a^2 + b^2 - 2abcostheta

Property of a dot product; x*x=x^2

Replace c^2, a^2, and b^2 with dot product

3. The attempt at a solution

c*c=a*a+b*b-2abcostheta

since c=a-b

c*c= (a-b) * (a-b)

c*c= (a*a-2a*b+b*b)

therefore

(a*a-2a*b+b*b) = a*a+b*b-2abcostheta

-2a*b=-2abcostheta

a*b=abcostheta

Does this look right?

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# Homework Help: How to Prove that A*B= |A||B|cos theta

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