Science:Math Exam Resources/Courses/MATH100/December 2013/Question 01 (d)

MATH100 December 2013

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Question 01 (d)
Short-Answer Question. Simplify your answer as much as possible and show your work. A bacteria culture initially contains 50 cells and grows at a rate proportional to its size. After an hour the population has increased to 250. Find an expression for the number of bacteria after t hours.

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
Try expressing this word problem as a differential equation: "[the culture] grows at a rate proportional to its size."

Hint 2
What is the solution to the differential equation ${\frac {dy}{dt}}=ky$?

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

Found a typo? Is this solution unclear? Let us know here. Please rate my easiness! It's quick and helps everyone guide their studies. Since the bacterial culture grows at a rate proportional to its size, we can say that ${\frac {dy}{dt}}=ky$, where $\displaystyle y(t)$ is the number of bacteria at time $\displaystyle t$ (in hours). The solution to this differential equation is: $\displaystyle y(t)=y(0)e^{kt}$ We are given that $\displaystyle y(0)=50$ and $\displaystyle y(1)=250$, so we can solve for $\displaystyle k$ as follows: ${\begin{aligned}y(1)&=y(0)e^{k\cdot 1}\\250&=50e^{k}\\5&=e^{k}\\k&=\ln 5\\\\\therefore y(t)&=50e^{\ln(5)t}\\&={\color {blue}50\cdot 5^{t}}\end{aligned}}$

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