a) $$\frac{(x + y)}{(x – y)}$$
b) $$\frac{(x + y)}{2xy}$$
c) $$\frac{2(x + y)}{xy}$$
d) $$\frac{2xy}{(x + y)}$$
Answer: b
Explanation: Required average
$$\eqalign{
& = \frac{{\left( {\frac{1}{x} + \frac{1}{y}} \right)}}{2} \cr
& = \frac{{x + y}}{{2xy}} \cr} $$
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