The Student Room Group
Uni students - Complete our survey >>
Uni students - Complete our survey >>
Uni students - Complete our survey >>

Differential equations

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?
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.
Reply 2
Avatar for laurawoods
laurawoods
OP
8
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? :smile:
this was the question (question no 7 on here)
http://www.london-oratory.org/maths/Solomon%20C4%20Worksheets/C4Edex/pdf_files/E4Qint_L.pdf
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? :smile:
this was the question (question no 7 on here)
http://www.london-oratory.org/maths/Solomon%20C4%20Worksheets/C4Edex/pdf_files/E4Qint_L.pdf


No it doesn't matter at all.
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? :smile:
this was the question (question no 7 on here)
http://www.london-oratory.org/maths/Solomon%20C4%20Worksheets/C4Edex/pdf_files/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 .
Reply 5
Avatar for laurawoods
laurawoods
OP
8
Original post by Mr M
No it doesn't matter at all.


hmmm . i never thought that way! thanks! :smile:

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you