Это старая версия документа!

$$\frac{1}{\sqrt{(x-b)^2+y^2}}\approx \frac{1}{r}+\frac{xb}{r^{3}}+\frac{\left(-r^{2}+3x^{2}\right)b^{2}}{2r^{5}}+\frac{\left(-3r^{2}+5x^{2}\right)xb^{3}}{2r^{7}}+O(b^4)$$

$$ 3\frac{a^{3}x\left(5x^{2}-3r^{2}\right)}{r^{7}} $$

$$ 3\frac{a^{3}\cos\theta\left(5\cos^{2}\theta-3\right)}{r^{5}} $$