1. Let $a<b<0$, and $a^{2}+b^{2}=4 a b$. Then the value of $\frac{a+b}{a-b}$ is ( ).
(A) $\sqrt{3}$
(B) $\sqrt{6}$
(C) 2
(D) 3
(2002, National Junior High School Mathematics Competition)
Prompt: Example 1. Answer: (A).

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