Consider the demand function $X(p)=100-10\sqrt{p}$, $(0 \leq p \leq 100)$. Determine the elasiticty.
$\frac{-10}{2\sqrt{p}}$
$\frac{1}{2}-\frac{\sqrt{p}}{20}$
$\frac{100\sqrt{p}-1000}{2p\sqrt{p}}$
$\frac{\sqrt{p}}{2\sqrt{p}-20}$
Correct:
$$\begin{align*}
\epsilon & = X'(p)\cdot \frac{p}{X(p)}\\
& = \frac{-10}{2\sqrt{p}}\cdot \frac{p}{100-10\sqrt{p}}\\
& = \frac{-10p}{200\sqrt{p}-20p}\\
&= \frac{-10\sqrt{p}}{200-20\sqrt{p}}\\
& = \frac{\sqrt{p}}{2\sqrt{p}-20}.
\end{align*}$$
Go on.
Wrong: $\epsilon \neq X'(p)$.
See Elasticity.
Wrong: $\frac{-10\sqrt{p}}{200-20\sqrt{p}}\neq \frac{1}{2}-\frac{\sqrt{p}}{20}$.
Try again.
Wrong: $\epsilon \neq X'(p)\cdot \frac{X(p)}{p}$.
See Elasticity.