1.79 Let $a=(\sqrt{5}+\sqrt{3})^{2}, b=(\sqrt{5}-\sqrt{3})^{-2}$. Then $(a+b)$ : $(a-b)$ is
(A) $\sqrt{5}: \sqrt{3}$.
(B) $5: 3$.
(C) $\sqrt{3}: \sqrt{5}$.
(D) $3: 5$.
(3rd "Five Sheep Cup" Junior High School Mathematics Competition, 1991)
[Solution] Let $b=(\sqrt{5}-\sqrt{3})^{-2}=\frac{1}{(\sqrt{5}-\sqrt{3})^{2}}$
$$
\begin{array}{c}
=\frac{(\sqrt{5}+\sqrt{3})^{2}}{4}=\frac{a}{4}, \\
(a+b):(a-b)=\left(a+\frac{a}{4}\right):\left(a-\frac{a}{4}\right)=5: 3 .
\end{array}
$$

Therefore, the answer is (B).