Answer
$\frac{1}{a^{-1}+b^{-1}}=\frac{ab}{a+b}$
Work Step by Step
The denominator was moved term-by-term to the numerator.
The correct procedure:
$\frac{1}{a^{-1}+b^{-1}}=\frac{1}{a^{-1}+b^{-1}}\frac{ab}{ab}=\frac{ab}{a+b}$
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