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A normal random variable \(x\) has mean \(\mu=10\) and standard deviation \(\sigma=2\). Find the probabilities of these \(x\) -values: a. \(x>13.5\) b. \(x<8.2\) c. \(9.4

Short Answer

Expert verified
The probability of \(x > 13.5\) is approximately 0.0401 or 4.01%. What is the probability of \(x < 8.2\)? The probability of \(x < 8.2\) is approximately 0.1841 or 18.41%. What is the probability of \(9.4 < x < 10.6\)? The probability of \(9.4 < x < 10.6\) is approximately 0.2358 or 23.58%.

Step by step solution

01

Standardize the normal variable by calculating the Z-score

We have the mean \(\mu = 10\) and standard deviation \(\sigma = 2\). To find the Z-score for \(x = 13.5\), use the formula: $$ Z = \frac{x - \mu}{\sigma} $$ Plug in the values of \(x\), \(\mu\), and \(\sigma\): $$ Z = \frac{13.5 - 10}{2} = 1.75 $$ So, \(Z = 1.75\).
02

Use the standard normal distribution table to find the probabilities

Now, we need to find the probability of \(P(Z > 1.75)\). This value can be found using a standard normal distribution table or online calculator. If using the table, we will find \(P(Z < 1.75)\) and then subtract that value from 1 to get \(P(Z > 1.75)\). From the table, we find that \(P(Z < 1.75) = 0.9599\). Therefore: $$ P(Z > 1.75) = 1 - P(Z < 1.75) = 1 - 0.9599 = 0.0401 $$ ## Part b: Find the probability of \(x < 8.2\) ##
03

Standardize the normal variable by calculating the Z-score

Calculate the Z-score for \(x = 8.2\) using the same formula: $$ Z = \frac{8.2 - 10}{2} = -0.9 $$ So, \(Z = -0.9\).
04

Use the standard normal distribution table to find the probabilities

Now, we need to find the probability of \(P(Z < -0.9)\). Using the standard normal distribution table or online calculator, we find that \(P(Z < -0.9) = 0.1841\). So, the probability of \(x < 8.2\) is \(0.1841\). ## Part c: Find the probability of \(9.4 < x < 10.6\) ##
05

Standardize the normal variable by calculating the Z-score

Calculate the Z-scores for \(x = 9.4\) and \(x = 10.6\) using the same formula: $$ Z_1 = \frac{9.4 - 10}{2} = -0.3\\ Z_2 = \frac{10.6 - 10}{2} = 0.3 $$ So, \(Z_1 = -0.3\) and \(Z_2 = 0.3\).
06

Use the standard normal distribution table to find the probabilities

Now, we need to find the probability of \(P(-0.3 < Z < 0.3)\). Using the standard normal distribution table or an online calculator, we can find the probabilities of \(P(Z < -0.3)\) and \(P(Z < 0.3)\), and then subtract to find the probability between them. $$ P(-0.3 < Z < 0.3) = P(Z < 0.3) - P(Z < -0.3) $$ From the table, we find that \(P(Z < -0.3) = 0.3821\) and \(P(Z < 0.3) = 0.6179\). Thus: $$ P(-0.3 < Z < 0.3) = 0.6179 - 0.3821 = 0.2358 $$ So, the probability of \(9.4 < x < 10.6\) is \(0.2358\).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Z-score Calculation
Understanding the Z-score is fundamental to interpreting data in a normal distribution. The Z-score represents the number of standard deviations a data point is from the mean. In mathematical terms, we calculate it using the formula:
\[ Z = \frac{x - \mu}{\sigma} \]
In this formula, \(x\) is the value of our normal random variable, \(\mu\) is the mean, and \(\sigma\) is the standard deviation. This normalizes different data sets to a standard scale, which allows for comparison, even when the original data have different units or variances.

In the example provided, calculating the Z-score transforms the original scores of \(x\) values into a standard scale where the mean (\(\mu\)) is zero and the standard deviation (\(\sigma\)) is one. This simplification is immensely helpful when trying to determine the probability of certain outcomes. Improving your grasp of Z-score concepts will bolster your confidence in tackling problems involving normal distributions.
Standard Normal Distribution Table
The standard normal distribution table, also known as a Z-table, is a reference that shows the area (or probability) to the left of a Z-score in a standard normal distribution. Since the total area under a normal curve equals 1, which corresponds to the probability of all outcomes, the table helps us to find the proportion or probability of observations that fall to the left (or right) of a Z-score.

For a right-tail event such as \(x > 13.5\), we compute \(P(Z > 1.75)\) by subtracting the table value for \(P(Z < 1.75)\) from 1. Such techniques are crucial when handling the tail ends of the distribution, and learning how to navigate the table is key to determining these probabilities effectively. Making ample use of the standard normal distribution table in practice will enhance your ability to compute probabilities for any Z-score.
Probability and Statistics
Probability and statistics are the foundational concepts that allow us to quantify uncertainty and make data-driven predictions. Probability quantifies the likelihood of an outcome or a set of outcomes, while statistics uses this notion to analyze and interpret data. In our normal distribution exercise, understanding probability helps us to calculate, for instance, how likely it is for a random variable to fall between two values.

With the calculated Z-scores, we can look at the cumulative distribution up to those points. By using probability (as read from the standard normal distribution table or calculated by software), we answer questions such as \(P(9.4 < x < 10.6)\), essentially finding the likelihood that a value falls within a specified interval of the mean. Grasping these principles will not only help in academic pursuits but also in real-world applications where decision-making often relies on statistical evidence and probabilistic assessments.

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