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Three 10-cm-long rods form an equilateral triangle. Two of the rods are charged to + 10 nC, the third to -10 nC. What is the electric field strength at the center of the triangle?

Answer in back of book: 1.08 * 10^5 N/C

By looking at the corresponding section, it gives the formula for the electric field strength in the plane that bisects a charged rod as:

Q/(4*pi*e_0*r*sqrt(r^2 + (L/2)^2)),

where e_0 is given to be 8.85*10^-12.

I thought that you would add the vertical and horizontal components of the 3 electric fields in the center due to each rod to get the net electric field. After some calculations, I get that the final answer should be 2 times the formula I gave above, but this does not give me the correct answer. What am I doing wrong? Please respond, thanks!