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Ch. 8 - Techniques of Integration
Hass - Thomas' Calculus 15th Edition
Hass15th EditionThomas' CalculusISBN: 9780137616077Not the one you use?Change textbook
Chapter 8, Problem 8.9.40

Annual rainfall The annual rainfall in inches for San Francisco, California, is approximately a normal random variable with mean 20.11 in. and standard deviation 4.7 in. What is the probability that next year’s rainfall will exceed 17 in.?

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1
Identify the random variable \( X \) representing the annual rainfall, which follows a normal distribution with mean \( \mu = 20.11 \) inches and standard deviation \( \sigma = 4.7 \) inches.
Standardize the value 17 inches to find the corresponding z-score using the formula: \[ Z = \frac{X - \mu}{\sigma} = \frac{17 - 20.11}{4.7} \]
Calculate the z-score to determine how many standard deviations 17 inches is below the mean rainfall.
Use the standard normal distribution table (or a calculator) to find the probability \( P(Z \leq z) \) corresponding to the calculated z-score.
Since the problem asks for the probability that rainfall exceeds 17 inches, compute \( P(X > 17) = 1 - P(Z \leq z) \).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Normal Distribution

The normal distribution is a continuous probability distribution characterized by a symmetric, bell-shaped curve defined by its mean and standard deviation. It models many natural phenomena, including rainfall amounts, allowing us to calculate probabilities for ranges of values.
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Standardization (Z-Score)

Standardization converts a normal random variable to a standard normal variable with mean 0 and standard deviation 1 using the formula Z = (X - μ) / σ. This transformation enables the use of standard normal tables or software to find probabilities.
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Probability Calculation for Normal Variables

To find the probability that a normal variable exceeds a certain value, we calculate the corresponding Z-score and then use the standard normal distribution to find the area to the right of that Z. This area represents the probability of exceeding the given value.
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