- #1
TheSodesa
- 224
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Homework Statement
Let ##X \sim Bin(n, p)## where ##n=20## and ##p=0.1##. Calculate ##P(|X-\mu| \leq \sigma)##.
Give your answer up to three decimal places.
Homework Equations
For a binomially distributed random variable, using moment generating functions we have:
\begin{equation}
\mu= E(X) = np
\end{equation}
\begin{equation}
\sigma^{2} = Var(X) = np(1-p)
\end{equation}
The probability density function is
\begin{equation}
f(x) = {n \choose x} p^{x}(1-p)^{n-x}
\end{equation}
The Attempt at a Solution
Now the asked probability looked a lot like Tsebyshev's inequality, but that just gave me a zero, and the electronic return system complained about it. It also gave me a hint:
First solve ##|X - \mu| \leq \sigma## and then calculate
[tex]
P(|X - \mu| \leq \sigma) = P(X=x_1 \text{ OR } X = x_2 ...)
[/tex]
I started out by solving for ##X##:
\begin{align*}
|X-\mu| \leq \sigma\\
\iff\\
-\sigma \leq X-\mu \leq \sigma\\
\iff\\
\mu-\sigma \leq X \leq \mu + \sigma\\
\iff\\
np - \sqrt{np(1-p)} \leq X \leq np + \sqrt{np(1-p)}\\
\iff\\
\stackrel{\approx 0.658}{2 - \sqrt{2(0.9)}} \leq X \leq \stackrel{\approx 3.342}{2 + \sqrt{2(0.9)}}\\
\end{align*}
Alright. Now I have some numerical values. But now what? I don't know what ##x_1##, ##x_2## etc. are in the hint. Are they referring to ##X=1##, ##X=2## and so on?
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