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## Homework Statement

## Homework Equations

## The Attempt at a Solution

can anyone enlighten me what the red and blue equations mean? i remembered something about the red eqn meaning unit vector in vector format, or scalar format is it?

and the blue eqn i have totally no idea what my prof is doing >< is he trying to say that del F is a dot product and since they are 0 because i =/= j, that means that cross product is 0???

help appreciated!

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