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# Differential equations Watch

1. A quantity has the value P at time t seconds and is decreasing at a rate
proportional to P .

Given that when t = 0, P = 400,
b find the value of a.
Given also that when t = 30, P = 100,
c find the value of P when t = 50.

Hello I had a question to ask : initially I didn't put -k in order to indicate that it is decreasing . But I still ended up with the answer they have given in the mark scheme? how come?
2. (Original post by laurawoods)
A quantity has the value P at time t seconds and is decreasing at a rate
proportional to P .

Given that when t = 0, P = 400,
b find the value of a.
Given also that when t = 30, P = 100,
c find the value of P when t = 50.

Hello I had a question to ask : initially I didn't put -k in order to indicate that it is decreasing . But I still ended up with the answer they have given in the mark scheme? how come?
Your k will be negative where their k is positive. The result is the same in the end.
3. (Original post by Mr M)
Your k will be negative where their k is positive. The result is the same in the end.
So is not compulsary to to put the - in front of the k when in the question you have got a decreasing rate?
this was the question (question no 7 on here)
http://www.london-oratory.org/maths/...s/E4Qint_L.pdf
4. (Original post by laurawoods)
So is not compulsary to to put the - in front of the k when in the question you have got a decreasing rate?
this was the question (question no 7 on here)
http://www.london-oratory.org/maths/...s/E4Qint_L.pdf
No it doesn't matter at all.
5. (Original post by laurawoods)
So is not compulsary to to put the - in front of the k when in the question you have got a decreasing rate?
this was the question (question no 7 on here)
http://www.london-oratory.org/maths/...s/E4Qint_L.pdf
think of it like this. Well more specifically. By putting K, you should be saying ( or thinking) this k represents a negative number. Whereas -k , you should be thinking k is a positive number ( constant) , so -k is a negative number and representing the decrease.

So what you did was implicity assume k was negative .
6. (Original post by Mr M)
No it doesn't matter at all.
hmmm . i never thought that way! thanks!

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