Science:Math Exam Resources/Courses/MATH103/April 2014/Question 01 (b) ii

MATH103 April 2014

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Question 01 (b) ii
Let $p(x)$ be a probability density function defined on the interval $[0,10]$. Determine whether the following statement is true or false: $0\leq \int _{0}^{10}tp(t)\mathrm {d} t\leq 10$.

Make sure you understand the problem fully: What is the question asking you to do? Are there specific conditions or constraints that you should take note of? How will you know if your answer is correct from your work only? Can you rephrase the question in your own words in a way that makes sense to you?

If you are stuck, check the hints below. Read the first one and consider it for a while. Does it give you a new idea on how to approach the problem? If so, try it! If after a while you are still stuck, go for the next hint.

Hint 1
What does the quantity $\int _{0}^{10}tp(t)\mathrm {d} t$ represent?

Hint 2
Start with $0\leq t\leq 10$. What can you say about $tp(t)$ in this case?

Hint 3
Conclude using $\int _{0}^{10}p(t)\mathrm {d} t=1$.

Checking a solution serves two purposes: helping you if, after having used all the hints, you still are stuck on the problem; or if you have solved the problem and would like to check your work. If you are stuck on a problem: Read the solution slowly and as soon as you feel you could finish the problem on your own, hide it and work on the problem. Come back later to the solution if you are stuck or if you want to check your work. If you want to check your work: Don't only focus on the answer, problems are mostly marked for the work you do, make sure you understand all the steps that were required to complete the problem and see if you made mistakes or forgot some aspects. Your goal is to check that your mental process was correct, not only the result.

Solution 1

Found a typo? Is this solution unclear? Let us know here. Please rate my easiness! It's quick and helps everyone guide their studies. The quantity $\int _{0}^{10}tp(t)\mathrm {d} t$ is the mean of a random quantity with probability density $p$. Since $p$ is a density on the interval $[0,10]$, the random quantity can take values in $[0,10]$. Thus, the mean is also a value in $[0,10]$. The statement is true.

Solution 2
Found a typo? Is this solution unclear? Let us know here. Please rate my easiness! It's quick and helps everyone guide their studies. Multiplying the inequality $0\leq t\leq 10$ by $p(t)$ yields $0\leq tp(t)\leq 10p(t)$. Integrating this gives $0\leq \int _{0}^{10}tp(t)\mathrm {d} t\leq 10\int _{0}^{10}p(t)\mathrm {d} t=10.$ The statement is true.