E(XY) – E(X)E(Y) σ(Χ)σ(Υ) 1. The correlation of X and Y is defined as px,Y Quickly show that if X and Y are independent, then px,y = 0.

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E(XY) – E(X)E(Y)
1. The correlation of X and Y is defined as px,y
if X and Y are independent, then px,y = 0.
Quickly show that
o(X)o(Y)
2. Let X be a random variable with P(X = k) = | for k = -1,0, 1. Let Y = X².
(a) What is the p.m.f. of Y?
(b) Are X and Y independent?
(c) Calculate E(XY) – E(X)E(Y).
(d) True or false (for any random variables X and Y): X and Y are independent if and only
if E(XY) – E(X)E(Y).
3. An exam consists of twenty multiple choice questions. Each question has five different possible
answers, only one of which is correct. A person randomly guesses on each question. What is
the variance of the number of questions that are correct?
4. Suppose ten people get on an elevator. There are eight buttons representing floors on this
elevator. Each person presses a random button. The elevator will only stop at a floor if at
least one person pressed that floor's button. What is probability the elevator stops at exactly
six floors? What is the variance of the number of floors the elevator will stop at?
5. A game costs $10 to play. In the game, you roll three fair six-sided dice. If you roll one six,
you win $5. If you roll two sixes, you win $10. If you roll three sixes, you win $K. What
value must K be so that the game is fair (the expected winnings are $0)?
Transcribed Image Text:E(XY) – E(X)E(Y) 1. The correlation of X and Y is defined as px,y if X and Y are independent, then px,y = 0. Quickly show that o(X)o(Y) 2. Let X be a random variable with P(X = k) = | for k = -1,0, 1. Let Y = X². (a) What is the p.m.f. of Y? (b) Are X and Y independent? (c) Calculate E(XY) – E(X)E(Y). (d) True or false (for any random variables X and Y): X and Y are independent if and only if E(XY) – E(X)E(Y). 3. An exam consists of twenty multiple choice questions. Each question has five different possible answers, only one of which is correct. A person randomly guesses on each question. What is the variance of the number of questions that are correct? 4. Suppose ten people get on an elevator. There are eight buttons representing floors on this elevator. Each person presses a random button. The elevator will only stop at a floor if at least one person pressed that floor's button. What is probability the elevator stops at exactly six floors? What is the variance of the number of floors the elevator will stop at? 5. A game costs $10 to play. In the game, you roll three fair six-sided dice. If you roll one six, you win $5. If you roll two sixes, you win $10. If you roll three sixes, you win $K. What value must K be so that the game is fair (the expected winnings are $0)?
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