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# P3 rates of change watch

1. Err, yeh - this is stupidly easy but I missed the lesson due to having a car crash and I havent really got the hang of it.

The area of a square is increasing at the rate of 7cm^2/sec. Find the rate of increase of the length of a side when this area is 100cm^2.

So: dA/dt = 7 - I need to find dx/dt.

dx/dt = dx/dA x dA/dt. Now what?
2. (Original post by imasillynarb)
Err, yeh - this is stupidly easy but I missed the lesson due to having a car crash and I havent really got the hang of it.

The area of a square is increasing at the rate of 7cm^2/sec. Find the rate of increase of the length of a side when this area is 100cm^2.

So: dA/dt = 7 - I need to find dx/dt.

dx/dt = dx/dA x dA/dt. Now what?
A = x^2
dA/dx = 2x , therefore; dx/dA = 1/(2x)
dA/dt = 7
x = 10 at the specified time.

dx/dt = (dx/dA) * (dA/dt)

dx/dt = (1/20) * (7)
dx/dt = 7/20

Ben
3. (Original post by Ben.S.)
A = x^2
dA/dx = 2x , therefore; dx/dA = 1/(2x)
dA/dt = 7
x = 10 at the specified time.

dx/dt = (dx/dA) * (dA/dt)

dx/dt = (1/20) * (7)
dx/dt = 7/20

Ben
thanks
4. P3!?
5. (Original post by fishpaste)
P3!?

Ben
6. (Original post by Ben.S.)

Ben
I've seen papers from last decade where it was on P1, last year it was on P2, now it's on P3?
7. (Original post by fishpaste)
I've seen papers from last decade where it was on P1, last year it was on P2, now it's on P3?
The chain rule is on Edexcel's P3, yes. It's pretty basic in itself, but lots of the things you need it for are not. There isn't much point teaching it until P3, when you need to integrate by substitution etc.

Ben
8. (Original post by Ben.S.)
The chain rule is on Edexcel's P3, yes. It's pretty basic in itself, but lots of the things you need it for are not. There isn't much point teaching it until P3, when you need to integrate by substitution etc.

Ben
What about related rates of change though? :|
9. (Original post by fishpaste)
What about related rates of change though? :|
yeah, i think i remember doing the chain rule in p2, but im unsure. it is pretty trivial though.
10. (Original post by fishpaste)
What about related rates of change though? :|
What do you mean? Partial derivatives? Or the type of thing above?

Ben
11. (Original post by elpaw)
yeah, i think i remember doing the chain rule in p2, but im unsure. it is pretty trivial though.
Yup.
12. (Original post by Ben.S.)
What do you mean? Partial derivatives? Or the type of thing above?

Ben
No just the old dv/dt = dv/dx * dx/dt sort of thing.
13. (Original post by elpaw)
yeah, i think i remember doing the chain rule in p2, but im unsure. it is pretty trivial though.
I don't think it's in Edexcel's P2. I'm pretty sure that sort of thing starts at P3.

Ben
14. (Original post by fishpaste)
No just the old dv/dt = dv/dx * dx/dt sort of thing.
Yeah - P3. I remember the pointless binomial expnsions bit in P2!

Ben
15. i have just looked at the syllabus... what the hell have they done? what is this C1 FP1 crap?

edit: oops, looking at the 2005 spec.
16. (Original post by elpaw)
i have just looked at the syllabus... what the hell have they done? what is this C1 FP1 crap?
Syllabus for what? At first sight they look like letters.

Ben
17. (Original post by elpaw)
i have just looked at the syllabus... what the hell have they done? what is this C1 FP1 crap?

edit: oops, looking at the 2005 spec.
That doesnt start until next year
18. (Original post by elpaw)
i have just looked at the syllabus... what the hell have they done? what is this C1 FP1 crap?
Next years syllabus. C stands for core i think. I dont understand why they have to change everything from year to year.
19. (Original post by elpaw)
i have just looked at the syllabus... what the hell have they done? what is this C1 FP1 crap?

edit: oops, looking at the 2005 spec.
I know Guz, it is really weird now - I'll be doing my Further Maths A-Level on OCR. At least there is a lot of flexibility with modules, and no coursework required. (phew)
20. (Original post by bono)
I know Guz, it is really weird now - I'll be doing my Further Maths A-Level on OCR. At least there is a lot of flexibility with modules, and no coursework required. (phew)
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