## Algebra and Trigonometry 10th Edition

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