Show that for any real numbers $\displaystyle a, b, c$ with $\displaystyle a < b < c$ we have

$\displaystyle PV \int^{\infty}_{-\infty} \frac{1}{(x-a)(x-b)(x-c)}dx =0.$

I know, in general that the Cauchy principal value of a doubly infinite integral of a function $\displaystyle f$ is

$\displaystyle PV \int_{-\infty}^{\infty}f(x)dx=\lim_{R \rightarrow \infty}\int_{-R}^Rf(x)dx$.

Also, here we have three consecutive real numbers. However, I do not see how we get the Cauchy principal value from all this. I need a bit of help on how this one.