1. In $\triangle A B C$, $\frac{2}{a}=\frac{1}{b}+\frac{1}{c}$. Then $\angle A$ ).
(A) must be an acute angle
(B) must be a right angle
(C) must be an obtuse angle
(D) none of the above answers is correct
$\begin{array}{l}\text { I. 1. A. } \\ \text { From } \frac{2}{a}=\frac{1}{b}+\frac{1}{c} \Rightarrow 2 b c=a(b+c) \text {. } \\ \text { Also, } b+c>a \text {, then } 2 b c>a^{2} \text {. } \\ \text { From }(b-c)^{2} \geqslant 0 \Rightarrow b^{2}+c^{2} \geqslant 2 b c \\ \Rightarrow b^{2}+c^{2}>a^{2} \\ \Rightarrow \angle A \text { must be an acute angle. }\end{array}$