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Calculus 2Open question
muroscamsey muroscamsey 2022-08-16

Rates of change, compounding rates and exponentiationI have a very (apologies if stupidly) simple question about rates of change that has been bugging me for some time. I can't work out whether it relates to my misunderstanding what a rate of change is, to my misapplying the method for calculating a rate of change or something else. I'm hoping somebody on here can help.
For how I define a rate of change, take as an example a population of 1000 items (e.g. bacteria). I observe this population and after an hour I count the size of the population and see that it has increased by 10% (to 1100). I might hypothesise that the population is growing at the rate of 10% per hour, and if, an hour later, I see that it has grown by 10% again (to 1,210) then I might decide to conclude that it is growing at 10% per hour.
So, a rate of change of "proportion x per hour" means "after one hour the population will have changed by proportion x". If, after 1 hour, my population of bacteria was not 1,100, and if not 1,210 after 2 hours, that would mean that the rate of change was not 10% per hour.
First question: Is this a fair definition of a rate of change?
So far so good and it's easy to calculate the population after any given time using a compound interest-type formula.
But whenever I read about continuous change something odd seems to happen. Given that "grows at the rate of 10% per hour" means (i.e. is just another way of saying) "after 1 hour the original population will have increased by 10%", why do textbooks state that continuous change should be measured by the formula:
P = P 0 e r t
And then give the rate of change in a form where this seems to give the wrong answer (i.e. without adjusting it to account for the continuously compounded growth)? I've seen many texts and courses where 10% per day continuous growth is calculated as (for my above example, after 1 day):
1000 e 1 0.1 = 1105.17
This contradicts the definition of a rate of change expressed as "x per unit of time" stated above. If I was observing a population of 1000 bacteria and observed it grow to a population of 1105 after 1 hour I should surely conclude that it was growing at the rate of 10.5% per hour.
I can get the idea of a continuous rate just fine, and it's easy to produce a continuous rate of change that equates to a rate of 10% per day as defined above (that's just ln 1.1). But I struggle to see how a rate of change that means a population grows by 10.5% in an hours means it is growing at 10% per hour. That's like saying if I lend you money at 1% interest per month I'd be charging you 12% per year.
So what's wrong here? Have I got the wrong end of the stick with my definition of a rate of change, would most people interpret a population increase of 10.5% in an hour as a 10% per hour growth rate, or is something else amiss?

Calculus 2Answered question
Aleseelomnl Aleseelomnl 2022-08-11

Rates of change
I’m having some trouble with part c) of the following questions,
a) What is the rate of change of the area A of a square with respect to its side x?
b) What is the rate of change of the area A of a circle with respect to its radius r?
c) Explain why one answer is the perimeter of the figure but the other answer is not.
So, knowing that if we have a square with side length x, then the area of the square as a function of its side is A ( x ) = x 2 . The perimeter as a function of the side is P ( x ) = 4 x. And the rate of change of the area wrt its side is d A d x = 2 x. With a circle, the area as a function of the radius is A ( r ) = π ( r 2 ). And the rate of change of the area wrt its radius is d A d r = 2 π ( r ). The circumference as a function of the radius is also C ( r ) = 2 π ( r ). Therefore it’s the circle that’s the figure with the rate of change of the area wrt its radius equal to its perimeter, and what I saw was that the square had a rate of change of area wrt its side equal to half the perimeter of the square, d A d x = 2 x = 4 x 2
I inscribed a circle in a square with radius equal to half the square’s side length and went through the same work and then arrived at this, A ( r ) = π ( x 2 ) 2 = π 4 x 2 and C ( x 2 ) = 2 π ( x 2 and that d A d r = π 2 x
Somehow in this example, I don’t think it’s correct because the same fact about the rate of change of area wrt radius being equal to perimeter doesn’t hold. I appreciate any help in explaining this, thank you.

Calculating rate of change is an essential skill for understanding the behavior of functions in mathematics. It is the rate at which a dependent variable changes with respect to an independent variable. It is found by taking the difference between two points and dividing it by the difference between the corresponding x-values. Rate of change can be used to predict trends of data sets and draw conclusions about the behavior of the function. Knowing the rate of change of a function can be beneficial when graphing and interpreting the behavior of the function. It can also be used to estimate the value of a function at a particular x-value. Understanding rate of change is key to success in mathematics.