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There are three charges : the first charge of +2Q locates at (0,0), the second charge of -Q locates at (0,d), the third charge of -Q locate (0,-d). Find the voltage at position x (X,0) when X>>>d. Use binomial theorem to expand the answer. Suppose that at x=2m, V= 160, find the approximate value of V at x=4. Assume that d <<<2m

I don't know how to insert a drawing in here. But according to the problem, the three charge would all on the y axis and the point where we have to find the voltage is gonna be on the x axis. I found:

V = (2Q/4*pi*epsilon) * (1/x - 1/sqrt{d^2+x^2})

Even after I use binomial expansion, I still couldn't cancel out d. I assume that somehow I have to cancel out d cause later on I have to estimate V without any exact value for d.

Can I anyone help me please. Thanks a lot!