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Differential equations

Hi, i need help wth this question. TIA x
(edited 1 year ago)

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Original post by issy_shep
Hi, i need help wth this question. TIA x


Presuming youre starting at a? So when it says rate of increase (change) what terms might be involved?
Reply 2
Original post by mqb2766
Presuming youre starting at a? So when it says rate of increase (change) what terms might be involved?

k & x
Original post by issy_shep
k & x


Assuming x is the population, how do you express the rate of change/increase? How do you represent the 0.5% of x? No idea what k is.
Reply 4
Original post by mqb2766
Assuming x is the population, how do you express the rate of change/increase? How do you represent the 0.5% of x? No idea what k is.

0.5x? Or 0.5kx?
Original post by issy_shep
0.5x? Or 0.5kx?


0.5x is a half of x and still no idea what k is. Theres also no differential term or an equation.

Do you have an example in your textbook (like radioactive decay or ...) which describes a simple differential equation?
https://math24.net/radioactive-decay.html
This obviously decreases, but it has some similarities with what you want.
Reply 6
Original post by mqb2766
0.5x is a half of x and still no idea what k is. Theres also no differential term or an equation.

Do you have an example in your textbook (like radioactive decay or ...) which describes a simple differential equation?
https://math24.net/radioactive-decay.html
This obviously decreases, but it has some similarities with what you want.

wait so what is the answer? Kx/2?
Original post by issy_shep
wait so what is the answer? Kx/2?


I cant help thinking you should have a good read of that section of your textbook rather than trying to guess. Youve not included a differential term or written something as an equation "=" and really there is no k in the answer (youve not said what it is) and x/2 is not 0.5% - think back to gcse about how you do percentages.
Reply 8
Dy/dx = 0.5x
Original post by issy_shep
Dy/dx = 0.5x


Its not right, but it is a differential equation at least. What do y and x represent and 0.5x is still not 0.5%. Why not try and try and describe what youre thinking?
Reply 10
Original post by mqb2766
Its not right, but it is a differential equation at least. What do y and x represent and 0.5x is still not 0.5%. Why not try and try and describe what youre thinking?

I'm thinking that k = 0.005 coz it's says that it increases by 0.5%
x = population size
And y = population of country
(edited 1 year ago)
Original post by issy_shep
I'm thinking that k = 0.005 coz it's says that it increases by 0.5%
x = population size
And y = population of country

0.005 as a multiplier is right. Whether you call it k is a red herring, but if x is the population size, then 0.005x is 0.5% of the peopulation.
However, look back to the radioactive decay example and think about the variables in the derivative on the left hand side of the equation and how they relate to the right. It should be similar for your case.
Reply 12
Original post by mqb2766
0.005 as a multiplier is right. Whether you call it k is a red herring, but if x is the population size, then 0.005x is 0.5% of the peopulation.
However, look back to the radioactive decay example and think about the variables in the derivative on the left hand side of the equation and how they relate to the right. It should be similar for your case.

I don't really understand that example. So the answer for a is dy/dx = 0.005x, correct?
Original post by issy_shep
I don't really understand that example. So the answer for a is dy/dx = 0.005x, correct?


No - where is time in your equation?
Reply 14
Original post by Muttley79
No - where is time in your equation?

Time?
Original post by issy_shep
Time?


Rate of increase ....
Reply 16
Original post by Muttley79
Rate of increase ....

Ohh so should it be dy/dt = 0.005t
Original post by issy_shep
Ohh so should it be dy/dt = 0.005t


Not quite.. Youve written the right hand side as 0.5% of time, rather than the population. Again, look at the radioactive decay example, or your textbook.
(edited 1 year ago)
Reply 18
Original post by mqb2766
Not quite.. Youve written the right hand side as 0.5% of time, rather than the population. Again, look at the radioactive decay example, or your textbook.

I'm just going round in circles omds. So dy/dx = 0.005xt ?
Original post by issy_shep
I'm just going round in circles omds. So dy/dx = 0.005xt ?


You really need to go over this section of the textbook/write your terms down clearly, but assuming x is the population then
0.005x
is 0.5% of the population.

The population x changes/is a function of time. You differentiate x with respect to time and that equals 0.5% of x just like the radioactive decay equation. There is no "y" assuming the population is denoted by x.

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