If `a\ne\p`, `b\ne\q`, `c\ne\r` and $ \begin{vmatrix} p & b & c \\ a & q & c \\ a & b & r \end{vmatrix} = 0 $, then show that `\frac{p}{p-a}+\frac{q}{q-b}+\frac{r}{r-c}=2`.