\[ x = \frac{-28 \pm \sqrt{28^2 + 4 \cdot 508}}{2} = \frac{-28 \pm \sqrt{3136 + 2032}}{2} = \frac{-28 \pm \sqrt{5168}}{2} \approx 10 \]

\[ x = \frac{-b}{2a} = \frac{-0}{2 \times -2} = 0 \quad \text{(conflict, no } x\text{-term)} \]
\[ \text{Max revenue at } x = \frac{-100}{2 \times (-2)} = \frac{100}{4} = 25 \]

\[ \frac{1}{x} + \frac{1}{x+1} = \frac{5}{6} \] Take LCM: \[ \begin{aligned} \frac{(x+1) + x}{x(x+1)} &= \frac{5}{6} \\ \frac{2x + 1}{x(x+1)} &= \frac{5}{6} \end{aligned} \]
\[ \begin{aligned} x &= \frac{7 \pm \sqrt{49 + 120}}{10} \\ &= \frac{7 \pm \sqrt{169}}{10} \\ &= \frac{7 \pm 13}{10} \end{aligned} \]

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