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The function f(x) represents a normal distribution whose standard deviation and mean are 1 and 5, respectively. The value of f(x) at x = 5 is

  • A.0.0
  • B.0.159
  • C.0.282
  • D.0.398

Answer: D

The value is 0.398.

The normal density is \(f(x) = \frac{1}{\sigma\sqrt{2\pi}}e^{-(x-\mu)^{2}/2\sigma^{2}}\). At the mean, \(x = \mu = 5\), the exponent is 0, so with \(\sigma = 1\):

\[ \begin{aligned} f(5) &= \frac{1}{\sqrt{2\pi}} \approx \frac{1}{2.5066} \\ &\approx 0.399 \end{aligned} \]

The density is greatest at the mean, so it cannot be 0.0. 0.159 is \(1/2\pi\), which omits the square root, and 0.282 is \(1/(2\sqrt{\pi})\), the value for \(\sigma = \sqrt{2}\), not 1.

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