15. If $a=1.69, b=1.73$ and $c=0.48$, find the value of
$$
\frac{1}{a^{2}-a c-a b+b c}+\frac{2}{b^{2}-a b-b c+a c}+\frac{1}{c^{2}-a c-b c+a b}
$$
15. Answer: 20
$$
\begin{array}{l}
\frac{1}{a^{2}-a c-a b+b c}+\frac{2}{b^{2}-a b-b c+a c}+\frac{1}{c^{2}-a c-b c+a b} \\
=\frac{1}{(a-b)(a-c)}+\frac{2}{(b-a)(b-c)}+\frac{1}{(c-a)(c-b)} \\
=\frac{c-b-2(c-a)-(a-b)}{(a-b)(b-c)(c-a)} \\
=\frac{-1}{(a-b)(b-c)}=20 .
\end{array}
$$