Electric Dipole: Calculate Electric Field at a Distance

[vipertongn]

Homework Statement
Two point charges likes those in the figure below are called an electric dipole. Show that the electric field at a distant point along the x-axis is given by $$E_{x}=\frac{4k_{e}qa}{x^3}$$
Figure: http://img300.imageshack.us/my.php?image=58ag9.png

Homework Equations

Electric field equation: $$E=\frac{k_{e}q}{r^2}$$

The Attempt at a Solution

I know that the total electric field at some point equals the vector sum of the electric fields of both charges. So...

-kq/r^2+kq/r^2?

From the solutions it puts in x-a and x+a for r values (x+a was orignally x-(-a)). I want to know why its subtracting the vector.

 

[berkeman]

Homework Statement
Two point charges likes those in the figure below are called an electric dipole. Show that the electric field at a distant point along the x-axis is given by $$E_{x}=\frac{4k_{e}qa}{x^3}$$
Figure: http://img300.imageshack.us/my.php?image=58ag9.png

Homework Equations

Electric field equation: $$E=\frac{k_{e}q}{r^2}$$

The Attempt at a Solution

I know that the total electric field at some point equals the vector sum of the electric fields of both charges. So...

-kq/r^2+kq/r^2?

From the solutions it puts in x-a and x+a for r values (x+a was orignally x-(-a)). I want to know why its subtracting the vector.

Did you check the signs on the q's?