CH5 CH6(Sections 1 through 5) Homework Problems

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1 CH5 CH6(Sections 1 through 5) Homework Problems 1. Part of HW #6: CH 5 P1. Let X be a random variable with probability density function f(x) = c(1 x ) 1 < x < 1 (a) What is the value of c? (b) What is the cumulative distribution function of X? P5. A filling station is supplied with gasoline once a week. If its weekly volume of sales in thousands of gallons is a random variable with probability density function f(x) = 5(1 x) 4 0 < x < 1 What need the capacity of the tank be so that the probability of the supply s being exhausted in a given week is.01? P7. The density function of X is given by If E[X] = 3, find a and b. 5 f(x) = a + bx 1 x 1 P10. Trains headed for destination A arrive at the train station at 15-minute intervals starting at 7 A.M., whereas trains headed for destination B arrive at 15- minute intervals starting at 7:05 A.M. (a) If a certain passenger arrives at the station at a time uniformly distributed between 7 and 8 A.M. and then gets on the first train that arrives, what proportion of time does he or she go to destination A? (b) What if the passenger arrives at a time uniformly distributed between 7:10 and 8:10 A.M.? 1

2 P11. A point is chosen at random on a line segment of length L. Interpret this statement and find the probability that the ratio of the shorter to the longer segment is less than 1 4. P14. Let X be a uniform (0,1) random variable. Compute E[X n ] by using Proposition.1 and then check the result by using the definition of expectation.. HW #7: CH 5 P16. The annual rainfall (in inches) in a certain region is normally distributed with µ=10 and σ =.4. What is the probability that starting with this year, it will take over 10 years before a year occurs having a rainfall of over 50 inches? What assumptions are you making? P18. Suppose that X is a normal random variable with mean 5. If P X > 9}=., approximately what is V ar(x)? P19. Lex X be a normal random variable with mean 1 and variance 4. Find the value of c such that P X > c}=.10. P1. Suppose that the height, in inches, of a 5-year-old man is a normal random variable with parameters µ=71 and σ =6.5. What percentage of 5-year-old men are over 6 feet inches tall? What percentage of men in the 6-footer club are over 6 foot 5 inches?

3 P. The width of a slot of a duralumin forging is (in inches) normally distributed with µ=.9000 and σ = The specification limits were given as.9000± (a) What percentage of forgings will be defective? (b) What is the maximum allowable value of σ that will permit no more than 1 in 100 defectives when the widths are normally distributed with µ=.9000 and σ? P3. One thousand independent rolls of a fair die will be made. Compute an approximation to the probability that number 6 will appear between 150 and 00 times inclusively. If number 6 appears exactly 00 times, find the probability that number 5 will appear less than 150 times. P5. Each item produced by a certain manufacturer is, independently, of acceptable quality with probability.95. Approximate the probability that at most 10 of the next 150 items produced are unacceptable. P7. In 10,000 independent tosses of a coin, the coin landed heads 5800 times. Is it reasonable to assume that the coin is not fair? Explain. 3. HW #8: CH 5 P3. The time (in hours) required to repair a machine is an exponentially distributed random variable with parameter λ = 1. What is (a) the probability that a repair time exceeds hours; (b) the conditional probability that a repair takes at least 10 hours, given that its duration exceeds 9 hours? 3

4 P34. Jones figures that the total number of thousands of miles that an auto can be driven before it would need to be junked is an exponential random variable with parameter 1. Smith has a used car that he claims has been driven only 10,000 0 miles. If Jones purchases the car, what is the probability that she would get at least 0,000 additional miles out of it? Repeat under the assumption that the lifetime mileage of the car is not exponentially distributed but rather is (in thousands of miles) uniformly distributed over (0, 40). P37. If X is uniformly distributed over (-1,1), find (a) P X > 1 }; (b) the density function of the random variable X. P40. If X is unformly distributed over (0, 1), find the density function of Y = e X T5. Use the result that for a nonnegative random variable Y, E[Y ] = 0 P Y > t}dt to show that for a nonnegative random variable X, E[X n ] = Hint: Start with E[X n ] = 0 nx n 1 P X > x}dx and make the change of variables t = x n. 0 P X n > t}dt T1. The median of a continuous random variable having distribution function F is that value m such that F (m) = 1. That is, a random variable is just as likely to be larger than its median as it is to be smaller. Find the median of X if X is (a) uniformly distributed over (a, b); (b) normal with parameters µ, σ ; (c) exponential with rate λ. 4

5 T4. Let ( ) X ν β Y = α Show that if X is a Weibull random variable with parameters ν, α, and β, then Y is an exponential random variable with parameter λ = 1 and vice versa. T9. Let X have probability density f X. Find the probability density functionof the random variable Y, defined by Y = ax + b. T30. Find the probability density function of Y = e X when X is normally distributed with parameters µ and σ. The random variable Y is said to have a lognormal distribution (since log Y has a normal distribution) with parameters µ and σ. 4. HW #9: CH 6 P1. Two fair dice are rolled. Find the joint probability mass function of X and Y when (a) X is the largest value obtained on any die and Y is the sum of the values; P. Suppose that 3 balls are chosen without replacement from an urn consisting of 5 white and 8 red balls. Let X i equal 1 if the ith ball selected is white, and let it equal. Give the joint probability mass function of (a) X 1, X ; 5

6 P8. The joint probability density function of X and Y is given by f(x, y) = c(y x )e y y x y, 0 < y < (a) Find c; (b) Find the marginal densities of X and Y. (c) Find E[X]. P10. The joint probability density function of X and Y is given by f(x, y) = e (x+y) 0 x <, 0 y < Find (a) P X < Y } and (b) P X < a}. P17. Three points X 1, X, X 3 are selected at random on a line L. What is the probability that X lies between X 1 and X 3? P0. The joint density of X and Y is given by f(x, y) = xe (x+y) x > 0, y > 0 Are X and Y independent? What if f(x, y) were given by f(x, y) = 0 < x < y, 0 < y < 1 P1. Let f(x, y) = 4xy 0 x 1, 0 y 1, 0 x + y 1 and let it equal. (a) Show that f(x, y) is a joint probability density function. (b) Find E[X] (c) Find E[Y ]. 6

7 P9. When a current I (measured in amperes) flows through a resistance R (measured in ohms), the power generated is given by W = I R (measuered in watts). Suppose that I and R are independent random variables with densities Determine the density function of W. f I (x) = 6x(1 x) 0 x 1 f R (x) = x 0 x 1 P4. The joint density function of X and Y is given by f(x, y) = xe x(y+1) x > 0, y > 0 (a) Find the conditional density of X, given Y = y, and that of Y, given X = x. P43. The joint density of X and Y is f(x, y) = c(x y )e x 0 x <, x y x Find the conditional distribution of Y, given X = x 7