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## Homework Statement

Show that the dot product of vectors a and b is equal to 1/4|a+b|^2 - 1/4|a-b|^2

## Homework Equations

a dot b = |a||b|cos(theta)

a dot b = a1b1 + a2b2 + ...

## The Attempt at a Solution

I've tried using the combination of the cosine law and those two above dot product equations and I have gotten no where. Any help would be greatly appreciated. (: