Math Problem Statement

A random variable X has the following probability function: X=x {0, 1, 2, 3, 4, 5, 6, 7} with P(x) {k, k, 2k, 2k, 3k, k^2, 2k^2, 7k^2 + k}. Tasks include: (i) Find k. (ii) Evaluate P(X<6) & P(0<X<5). (iii) Determine the distribution function of X.

Solution

Let's analyze and solve the problem step by step:

Problem Statement:

The table gives the probability distribution of a random variable XX, with the probability function values P(X=x)P(X = x) listed for x=0,1,2,3,4,5,6,7x = 0, 1, 2, 3, 4, 5, 6, 7. The values of P(X=x)P(X = x) depend on the constant kk, and we are tasked with:

  1. Finding kk.
  2. Evaluating P(X<6)P(X < 6) and P(0<X<5)P(0 < X < 5).
  3. Determining the cumulative distribution function (CDF) F(x)F(x).

Step 1: Find kk

The sum of all probabilities in a probability distribution must equal 1: P(X=0)+P(X=1)+P(X=2)+P(X=3)+P(X=4)+P(X=5)+P(X=6)+P(X=7)=1.P(X = 0) + P(X = 1) + P(X = 2) + P(X = 3) + P(X = 4) + P(X = 5) + P(X = 6) + P(X = 7) = 1.

Substitute the given probabilities: k+k+2k+2k+3k+k2+2k2+7k2+k=1.k + k + 2k + 2k + 3k + k^2 + 2k^2 + 7k^2 + k = 1.

Combine like terms: 5k+10k2=1.5k + 10k^2 = 1.

Divide through by kk (assuming k0k \neq 0): 5+10k=1k.5 + 10k = \frac{1}{k}.

Multiply through by kk: 5k+10k2=1.5k + 10k^2 = 1.

Rearrange into standard quadratic form: 10k2+5k1=0.10k^2 + 5k - 1 = 0.

Solve this quadratic equation using the quadratic formula: k=b±b24ac2a,k = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}, where a=10a = 10, b=5b = 5, c=1c = -1.

Substitute: k=5±524(10)(1)2(10).k = \frac{-5 \pm \sqrt{5^2 - 4(10)(-1)}}{2(10)}. k=5±25+4020.k = \frac{-5 \pm \sqrt{25 + 40}}{20}. k=5±6520.k = \frac{-5 \pm \sqrt{65}}{20}.

Take the positive root (since k>0k > 0): k=5+6520.k = \frac{-5 + \sqrt{65}}{20}.


**Step 2: Evaluate P(X<6)P(X < 6) and P(0<X<5)P(0 < X < 5)

(a) P(X<6)P(X < 6):

This means summing probabilities for X=0,1,2,3,4,5X = 0, 1, 2, 3, 4, 5: P(X<6)=P(X=0)+P(X=1)+P(X=2)+P(X=3)+P(X=4)+P(X=5).P(X < 6) = P(X = 0) + P(X = 1) + P(X = 2) + P(X = 3) + P(X = 4) + P(X = 5). P(X<6)=k+k+2k+2k+3k+k2=9k+k2.P(X < 6) = k + k + 2k + 2k + 3k + k^2 = 9k + k^2.

(b) P(0<X<5)P(0 < X < 5):

This means summing probabilities for X=1,2,3,4X = 1, 2, 3, 4: P(0<X<5)=P(X=1)+P(X=2)+P(X=3)+P(X=4).P(0 < X < 5) = P(X = 1) + P(X = 2) + P(X = 3) + P(X = 4). P(0<X<5)=k+2k+2k+3k=8k.P(0 < X < 5) = k + 2k + 2k + 3k = 8k.


Step 3: Determine the Cumulative Distribution Function (CDF) F(x)F(x)

The CDF F(x)F(x) is defined as: F(x)=P(Xx).F(x) = P(X \leq x).

Compute F(x)F(x) step by step for each xx:

  • F(0)=P(X=0)=k.F(0) = P(X = 0) = k.
  • F(1)=P(X=0)+P(X=1)=k+k=2k.F(1) = P(X = 0) + P(X = 1) = k + k = 2k.
  • F(2)=F(1)+P(X=2)=2k+2k=4k.F(2) = F(1) + P(X = 2) = 2k + 2k = 4k.
  • F(3)=F(2)+P(X=3)=4k+2k=6k.F(3) = F(2) + P(X = 3) = 4k + 2k = 6k.
  • F(4)=F(3)+P(X=4)=6k+3k=9k.F(4) = F(3) + P(X = 4) = 6k + 3k = 9k.
  • F(5)=F(4)+P(X=5)=9k+k2.F(5) = F(4) + P(X = 5) = 9k + k^2.
  • F(6)=F(5)+P(X=6)=9k+k2+2k2=9k+3k2.F(6) = F(5) + P(X = 6) = 9k + k^2 + 2k^2 = 9k + 3k^2.
  • F(7)=F(6)+P(X=7)=9k+3k2+7k2=9k+10k2.F(7) = F(6) + P(X = 7) = 9k + 3k^2 + 7k^2 = 9k + 10k^2.

Since F(7)=1F(7) = 1, this serves as a verification for the value of kk.


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Math Problem Analysis

Mathematical Concepts

Probability Distributions
Cumulative Distribution Functions (CDF)
Algebraic Manipulations
Quadratic Equations

Formulas

Sum of probabilities in a distribution: ΣP(X=x) = 1
Quadratic equation: ax^2 + bx + c = 0
Cumulative Distribution Function: F(x) = P(X ≤ x)

Theorems

Normalization of Probability
Quadratic Formula

Suitable Grade Level

Grades 11-12 or undergraduate