[Oromis263]

(Original post by kenau)
Hi Cathay
I only left one type of question don't know how to do it...
Can anyone help me the JUN 10 question 7c and 8?

I'll start you off on question 8. To start with, you separate the variables:



Continued inside spoiler, but give the rest of part A a go before looking.

Spoiler:

Show

Then from there you sub in when x=80 and t=0 to find c, then just rearrange the equations so you have x = f(t), as follows:

[ADA93]

Can anyone help me with Q4bi) on C4 Jan 2012? Why does the mark scheme do (e^1/8 t) ^2 = 500000/500? Thanks.

[hash007]

I don't recall being taught this, yet it has cropped up in a few past papers.

A giant snowball is melting. The snowball can be modelled as a sphere whose
surface area is decreasing at a constant rate with respect to time. The surface area of
the sphere is A cm2 at time t days after it begins to melt.

(a) Write down a differential equation in terms of the variables A and t and a constant k,
where k > 0 , to model the melting snowball.

How do you know what to do?

[Dangerous Theory]

(Original post by hash007)
I don't recall being taught this, yet it has cropped up in a few past papers.

A giant snowball is melting. The snowball can be modelled as a sphere whose
surface area is decreasing at a constant rate with respect to time. The surface area of
the sphere is A cm2 at time t days after it begins to melt.

(a) Write down a differential equation in terms of the variables A and t and a constant k,
where k > 0 , to model the melting snowball.

How do you know what to do?

surface area of a sphere, according to the formula book, is 4pir^2. i woold say something like dA /dT = 4pir^2t?

This was posted from The Student Room's Android App on my GT-I9300

[f1mad]

(Original post by Dangerous Theory)
surface area of a sphere, according to the formula book, is 4pir^2. i woold say something like dA /dT = 4pir^2t?

This was posted from The Student Room's Android App on my GT-I9300

No.

dA/dt = -k

[Oromis263]

(Original post by Dangerous Theory)
surface area of a sphere, according to the formula book, is 4pir^2. i woold say something like dA /dT = 4pir^2t?

This was posted from The Student Room's Android App on my GT-I9300

I think what I would put is:



if you put the t in before you separate the variables, you're saying that the rate is 0 when t=0, and that the rate gets larger with time, which it shouldn't as it is proportional to the surface area, which decreases with time, and hence the rate should decrease.

(Original post by hash007)
I don't recall being taught this, yet it has cropped up in a few past papers.

A giant snowball is melting. The snowball can be modelled as a sphere whose
surface area is decreasing at a constant rate with respect to time. The surface area of
the sphere is A cm2 at time t days after it begins to melt.

(a) Write down a differential equation in terms of the variables A and t and a constant k,
where k > 0 , to model the melting snowball.

How do you know what to do?

[f1mad]

(Original post by Oromis263)
I think what I would put is:



if you put the t in before you separate the variables, you're saying that the rate is 0 when t=0, and that the rate gets larger with time, which it shouldn't as it is proportional to the surface area, which decreases with time, and hence the rate should decrease.

I see you edited your post about 3 times .

The snowball can be modelled as a sphere whose surface area is decreasing at a constant rate with respect to time.

I.e -k

[Oromis263]

(Original post by f1mad)
I see you edited your post about 3 times .

The snowball can be modelled as a sphere whose surface area is decreasing at a constant rate with respect to time.

I.e -k

I read it at first as proportional to the surface area, I think something similar was on a M2 paper I just did and yeah, forgot the decreasing as well. All in all, was quite a major fail for me.. :P

Especially as I've already done that past paper a couple of weeks ago and got 75 ¬¬

[hash007]

(Original post by Oromis263)
I read it at first as proportional to the surface area, I think something similar was on a M2 paper I just did and yeah, forgot the decreasing as well. All in all, was quite a major fail for me.. :P

Especially as I've already done that past paper a couple of weeks ago and got 75 ¬¬

Have you done the June 11 C4 paper, the last question is a killer!

[Oromis263]

(Original post by hash007)
Have you done the June 11 C4 paper, the last question is a killer!

I didn't find it too bad, as I felt the first question set you up quite a lot, and from there the last 9 marks were just manipulation. However, I find that my calculus is usually pretty strong, and I like the open ended questions a lot. :P

[hash007]

(Original post by Oromis263)
I didn't find it too bad, as I felt the first question set you up quite a lot, and from there the last 9 marks were just manipulation. However, I find that my calculus is usually pretty strong, and I like the open ended questions a lot. :P

Yeah I got most of the marks for the last question, just couldn't get it into the form they wanted. Didn't pick up a single mark on the snowball question , I hate rates of change and differential equations. It's kinda hard for me to see what maths I need to do when they start being all wordy and putting it in context. I just want the numbers!

Got 62/75 which is A*, can't believe the grade boundaries were 58/75 for A*

[Oromis263]

(Original post by hash007)
Yeah I got most of the marks for the last question, just couldn't get it into the form they wanted. Didn't pick up a single mark on the snowball question , I hate rates of change and differential equations. It's kinda hard for me to see what maths I need to do when they start being all wordy and putting it in context. I just want the numbers!

Got 62/75 which is A*, can't believe the grade boundaries were 58/75 for A*

The examiners report for that paper makes quite an interesting read, I couldn't believe how low the boundaries were either! :P

[jammysmt]

Sorry if someone has already explained, but could somebody help me with question 7c, June 2010.

I don't know how you can work out both solutions, I only come out with one and the mark scheme seems to be confusing. I understand that you need to manipulate the info given, that AC=PB but HOW do you know to use BOTH AC=PB AND AP=BC. Surely they should given the same result? Help appreciated.

[sii217]

Anyone else who suddenly can't revise? haha
After pretty much 3 weeks going over stuff for the AS resits, and it all going into my brain nicely suddenly nothings sticking. I'm just sitting looking at a textbook with my mind wandering..

[f1mad]

(Original post by hash007)
Have you done the June 11 C4 paper, the last question is a killer!

What was the last question? A differential equation?

[hash007]

(Original post by f1mad)
What was the last question? A differential equation?

Yeah

[sydney07]

Hey, just wondering as I'm nearing finishing all the papers, is there a specimen paper for core 4 does anyone know? And if there is, please could someone leave a link (:

Thank you

[Ryanxclarke]

Maths question.bmp

I was just wondering if someone could help me with, question 1 from Jan 08.

Thanks

[f1mad]

(Original post by Ryanxclarke)
Maths question.bmp

I was just wondering if someone could help me with, question 1 from Jan 08.

Thanks

You need to get the LHS identical to the RHS then compare the numerator to find k.

[originalsteph]

hey could someone help me with the January 2012 question 4b I have attached the QP