diff --git a/src/chapters/6/sections/means_medians_modes_moments/index.tex b/src/chapters/6/sections/means_medians_modes_moments/index.tex index 1f290f4f..7280c601 100644 --- a/src/chapters/6/sections/means_medians_modes_moments/index.tex +++ b/src/chapters/6/sections/means_medians_modes_moments/index.tex @@ -4,3 +4,5 @@ \subsection{problem 1} \input{problems/1} \subsection{problem 2} \input{problems/2} +\subsection{problem 3} +\input{problems/3} diff --git a/src/chapters/6/sections/means_medians_modes_moments/problems/3.tex b/src/chapters/6/sections/means_medians_modes_moments/problems/3.tex new file mode 100644 index 00000000..2de38f3b --- /dev/null +++ b/src/chapters/6/sections/means_medians_modes_moments/problems/3.tex @@ -0,0 +1,18 @@ +Let k be the median, and F be the CDF of X, such that +\[F(k) = 1/2\] +since X is continuous from 1 onwards, and 0 otherwise, +\begin{align*} +F(k) &= \int_{-\infty}^{k}{\frac{a}{x^{a+1}}} \\ +&= \int_{1}^{k}{\frac{a}{x^{a+1}}} \\ +&= 1 - k^{-a} +\end{align*} +It follows that +\begin{align*} +1-k^{-a} &= \frac{1}{2} \\ +k &= \sqrt[a]{2} +\end{align*} +The median is therefore \(\sqrt[a]{2}\). \\ + +Let $f$ be the PDF of X, then, +\[f'(x) = -a(a+1)x^{-a-2}\] +since $f'(x) \le 0$ for all $x \in [1,\infty)$, $f(x)$ is strictly decreasing between 1 and \(\infty\) and 0 otherwise, $f(x)$ has maximum at 1, so 1 is the mode.