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Brrunno [24]
3 years ago
9

For X a binomial random variable with n=5 and p=1/4, answer the following

Mathematics
1 answer:
mash [69]3 years ago
7 0
Recall that for X\sim\mathrm{Bin}(n,p), i.e. a random variable X following a binomial distribution over n trials and with probability parameter p,

\mathbb P(X=x)=f_X(x)=\begin{cases}\dbinom nx p^x(1-p)^{n-x}&\text{for }x\in\{0,1,\ldots,n\}\\\\0&\text{otherwise}\end{cases}

So you have

\mathbb P(X=2)=\dbinom52\left(\dfrac14\right)^2\left(1-\dfrac34\right)^{5-2}=\dfrac{135}{512}\approx0.26

\mathbb P(2\le X\le4)=\displaystyle\sum_{x=2}^4\mathbb P(X=x)
=\dbinom52\left(\dfrac14\right)^2\left(1-\dfrac14\right)^{5-2}+\dbinom52\left(\dfrac14\right)^3\left(1-\dfrac14\right)^{5-3}+\dbinom52\left(\dfrac14\right)^4\left(1-\dfrac14\right)^{5-4}
=\dfrac{375}{1024}\approx0.37

The expected value of X\sim\mathrm{Bin}(n,p) is simply np, while the standard deviation is \sqrt{np(1-p)}. In this case, they are \dfrac54=1.25 and \sqrt{\dfrac{15}{16}}\approx0.97, respectively.
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