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## Homework Statement

An equilateral triangle has a point charge +q at each of the three vertices (A, B, C). Another point charge Q is placed at D, the midpoint of the side BC. What is the charge Q if the total electric force on the charge at A due to the charges at B, C, and D is zero? (Use any variable or symbol stated above as necessary.)

Image: http://www.webassign.net/grr/p16-24.gif

Q= _______

## Homework Equations

kq/r^2

## The Attempt at a Solution

The total net force on A is zero.

Let's say the length of AB and AC are d so that the distance of B and C to A are d. Now, we have to find the distance between Q and A. Well, Q is placed in the midpoint of BC, so that we can bisect the triangle by drawing a line through A and Q. That makes a 30-60-90 triangle in which the hypotenuse is d. Using trigonometry, we know that QB and QC are d/2, making QA (d*sqrt(3))/2.

Coulomb's law:

[ k(+q)(+q)/d² ] + [ k(+q)(+q)/d² ] + [ k(Q)(+q)/((d*sqrt(3))/2)² ]

To make the math easier, take out the common factor k(+q) and solve ((d*sqrt(3))/2)

k(+q) [ (+q)/d² + (+q)/d² +(Q)/(3d²/4) ]

So, +2q/d² + (Q)/(3d²/4) = 0

+2q/d² = -(Q)/(3d²/4)

Multiply 3d² and divide 4 to the other side

+2q(3d²)/4d² = -Q

d² cancels out, multiply both sides by -1

Q = -(3/2)q

Webassign says it's wrong. Can you think of any way I can rearrange it? Or is it done entirely wrong?