Simple three charges problem in 1D

[zimo]

Homework Statement

Two charges are on the X axis as follows: q1=3x10^-9 C at x=0.4metre and q2=5x10^-9 C at x=-0.2m. What is the size and direction of the sum of forces are on third charge q3=-2x10^-9 C at x=0?

Homework Equations

F=k|q1q2|/r^2

The Attempt at a Solution

By my calculation I achieved F(13)=3.375x10^6 N in the +x direction, and F(23)=2.247x10^6 N in the -x direction, and the sum of forces is 1.128x10^6 N to the +x direction. But the book says otherwise...

 

[Dick]

Something is a bit off, when I compute F(13) I get a very different exponent for the force. 3.37*10^(-7) N.

 

[zimo]

oh, darn