Calculating pdf from a given cdf

Calculating pdf from a given cdf

IB Year 5 | Grade 11

The continuous random variable \(X\) is such that

\[P(X > x) = \begin{cases} 1 & , & x \le 0 \\ k(3 - x)^3 & , & 0 < x < 3 \\ 0 & , & x \ge 3 \end{cases}\]

Show that \(\mathrm{P}(X > 1) = \frac{8}{27}\).

Find the probability density function of \(X\), and hence find \(\mathrm{E}(X)\).

Solution:

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Answer:\(\mathrm{P}(X>1)=\frac8{27}\). \(\mathrm{f}(x)=\frac{(3-x)^2}{9}\) for \(0<x<3\), and \(0\) otherwise. \(\mathrm{E}(X)=\frac34\).

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