Math Problem Statement

  1. The probability distribution table for X and Y is presented below. Y\X 0 1 2 0 0.05 0.1 0.03 1 0.21 0.11 0.19 2 0.08 0.15 0.08 1-a) compute COV(X 2 , Y)

Solution

To compute COV(X2,Y)\text{COV}(X^2, Y), we follow these steps:

Step 1: Recall the formula for covariance

The covariance between two random variables X2X^2 and YY is given by: [ \text{COV}(X^2, Y) = E[X^2 Y] - E[X^2] E[Y] ] We need to compute three things:

  1. E[X2Y]E[X^2 Y] — the expected value of the product of X2X^2 and YY,
  2. E[X2]E[X^2] — the expected value of X2X^2,
  3. E[Y]E[Y] — the expected value of YY.

Step 2: Calculate E[X2Y]E[X^2 Y]

Using the joint probability distribution table, we compute E[X2Y]E[X^2 Y] as: E[X2Y]=xy(x2yP(X=x,Y=y))E[X^2 Y] = \sum_x \sum_y (x^2 \cdot y \cdot P(X=x, Y=y)) We apply the table values.

For X=0X = 0:

  • (020P(0,0))=0200.05=0(0^2 \cdot 0 \cdot P(0,0)) = 0^2 \cdot 0 \cdot 0.05 = 0
  • (021P(0,1))=0210.1=0(0^2 \cdot 1 \cdot P(0,1)) = 0^2 \cdot 1 \cdot 0.1 = 0
  • (022P(0,2))=0220.03=0(0^2 \cdot 2 \cdot P(0,2)) = 0^2 \cdot 2 \cdot 0.03 = 0

For X=1X = 1:

  • (120P(1,0))=1200.21=0(1^2 \cdot 0 \cdot P(1,0)) = 1^2 \cdot 0 \cdot 0.21 = 0
  • (121P(1,1))=1210.11=0.11(1^2 \cdot 1 \cdot P(1,1)) = 1^2 \cdot 1 \cdot 0.11 = 0.11
  • (122P(1,2))=1220.19=0.38(1^2 \cdot 2 \cdot P(1,2)) = 1^2 \cdot 2 \cdot 0.19 = 0.38

For X=2X = 2:

  • (220P(2,0))=2200.08=0(2^2 \cdot 0 \cdot P(2,0)) = 2^2 \cdot 0 \cdot 0.08 = 0
  • (221P(2,1))=2210.15=40.15=0.6(2^2 \cdot 1 \cdot P(2,1)) = 2^2 \cdot 1 \cdot 0.15 = 4 \cdot 0.15 = 0.6
  • (222P(2,2))=2220.08=420.08=0.64(2^2 \cdot 2 \cdot P(2,2)) = 2^2 \cdot 2 \cdot 0.08 = 4 \cdot 2 \cdot 0.08 = 0.64

Now, summing all the terms: E[X2Y]=0+0+0+0+0.11+0.38+0+0.6+0.64=1.73E[X^2 Y] = 0 + 0 + 0 + 0 + 0.11 + 0.38 + 0 + 0.6 + 0.64 = 1.73

Step 3: Calculate E[X2]E[X^2]

Now, calculate the expected value of X2X^2: E[X2]=x(x2P(X=x))E[X^2] = \sum_x (x^2 \cdot P(X=x))

For X=0X = 0:

  • 02P(X=0)=02(0.05+0.1+0.03)=00^2 \cdot P(X=0) = 0^2 \cdot (0.05 + 0.1 + 0.03) = 0

For X=1X = 1:

  • 12P(X=1)=12(0.21+0.11+0.19)=10.51=0.511^2 \cdot P(X=1) = 1^2 \cdot (0.21 + 0.11 + 0.19) = 1 \cdot 0.51 = 0.51

For X=2X = 2:

  • 22P(X=2)=22(0.08+0.15+0.08)=40.31=1.242^2 \cdot P(X=2) = 2^2 \cdot (0.08 + 0.15 + 0.08) = 4 \cdot 0.31 = 1.24

Now, summing these values: E[X2]=0+0.51+1.24=1.75E[X^2] = 0 + 0.51 + 1.24 = 1.75

Step 4: Calculate E[Y]E[Y]

Now, calculate the expected value of YY: E[Y]=y(yP(Y=y))E[Y] = \sum_y (y \cdot P(Y=y))

For Y=0Y = 0:

  • 0P(Y=0)=0(0.05+0.21+0.08)=00 \cdot P(Y=0) = 0 \cdot (0.05 + 0.21 + 0.08) = 0

For Y=1Y = 1:

  • 1P(Y=1)=1(0.1+0.11+0.15)=10.36=0.361 \cdot P(Y=1) = 1 \cdot (0.1 + 0.11 + 0.15) = 1 \cdot 0.36 = 0.36

For Y=2Y = 2:

  • 2P(Y=2)=2(0.03+0.19+0.08)=20.30=0.602 \cdot P(Y=2) = 2 \cdot (0.03 + 0.19 + 0.08) = 2 \cdot 0.30 = 0.60

Now, summing these values: E[Y]=0+0.36+0.60=0.96E[Y] = 0 + 0.36 + 0.60 = 0.96

Step 5: Calculate the covariance

Now, substitute everything into the covariance formula: [ \text{COV}(X^2, Y) = E[X^2 Y] - E[X^2] E[Y] = 1.73 - (1.75 \cdot 0.96) ] COV(X2,Y)=1.731.68=0.05\text{COV}(X^2, Y) = 1.73 - 1.68 = 0.05

Thus, the covariance COV(X2,Y)\text{COV}(X^2, Y) is 0.05\boxed{0.05}.

Would you like further clarification or details on any step?


Here are 5 related questions for further practice:

  1. How would you calculate E[XY]E[XY] using the same table?
  2. What is the variance of XX?
  3. How would you find COV(X,Y)\text{COV}(X, Y) from the same table?
  4. What is the joint probability distribution function?
  5. How can you find the marginal distribution of XX?

Tip: Covariance is a measure of how two random variables change together. If it's positive, they tend to increase together, and if it's negative, one increases while the other decreases.

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Math Problem Analysis

Mathematical Concepts

Probability
Covariance
Expected Value

Formulas

COV(X^2, Y) = E[X^2 Y] - E[X^2] E[Y]
E[X^2 Y] = Σ_x Σ_y (x^2 ⋅ y ⋅ P(X=x, Y=y))
E[X^2] = Σ_x (x^2 ⋅ P(X=x))
E[Y] = Σ_y (y ⋅ P(Y=y))

Theorems

Law of Expected Value
Covariance Formula

Suitable Grade Level

Grades 11-12