Wzór podany w zadaniu:

$E=\frac{2kQlx}{{\left(x^2-\frac{l^2}{4}\right)}^2}$  

$E=\frac{2kQlx}{{\left(x^2\left(1-\frac{l^2}{4x^2}\right)\right)}^2}$

$E=\frac{2kQlx}{x^4{\left(1-\frac{l^2}{4x^2}\right)}^2}$

$E=\frac{2kQl}{x^3{\left(1-\frac{l^2}{4x^2}\right)}^2}$

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$x\text{>>}l\Rightarrow \frac{l}{x}\to 0\Rightarrow \frac{l^2}{4x^2}\to 0$  

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$E=\frac{2kQl}{x^3{\left(1-0\right)}^2}$

$E=\frac{2kQl}{x^3}$

Odp. C.