Question:

In a Poisson Distribution, if mean = e, then P(x) is given by ____________.

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Always start by writing down the general formula and then substitute the given parameters. Simplifying using basic exponent rules easily reveals the matching option.
  • $\exp(x-e)/x!$
  • $\exp(e-x)/e!$
  • $\exp(x-e)/e!$
  • $\exp(-x-e)/x!$
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The Correct Option is A

Solution and Explanation

Step 1: Understanding the Concept:
The Poisson distribution is a discrete probability distribution that models the number of events occurring in a fixed interval of time or space.
Key Formula or Approach:
The probability mass function of a Poisson distribution with parameter (mean) $\lambda$ is: \[ P(X = x) = \frac{e^{-\lambda} \lambda^x}{x!}, \quad x = 0, 1, 2, \dots \]

Step 2: Detailed Explanation:

Given that the mean of the distribution is: \[ \lambda = e \] Substitute $\lambda = e$ into the Poisson probability mass function: \[ P(X = x) = \frac{e^{-e} e^x}{x!} \] Combine the exponential terms in the numerator using exponents laws ($e^a \cdot e^b = e^{a+b}$): \[ P(X = x) = \frac{e^{x-e}}{x!} \] Rewriting this in functional notation: \[ P(X = x) = \frac{\exp(x-e)}{x!} \]

Step 3: Final Answer:

The derived probability mass function is $\exp(x-e)/x!$, which matches Option (A).
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