Electrostatics - Coulomb's Law

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jinksys
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I'm doing example 2.1 in Griffith's Electrodynamics book. Can someone explain where the cos(theta) comes from in the formula for dE? The formula is on the first image: Here.
 

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SOLVED:

I see what's going on. If you add the r_hat vectors you end up with 2(z/r)z_hat. z/r=cos(theta). So we end up with 2cos(theta)z_hat.
 
that looks so hard. are you supposed to figure out that formula all by yourself
 
bael said:
that looks so hard. are you supposed to figure out that formula all by yourself

No, the formula is given to you in a general form.
 

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