STEP III 2012 Discussion Thread

Original post by fruktas

In part ii, I am wondering, since the question said 'Find the values of.... in the two cases that arise., is it enough for state that it is 1/-1 for odd/even n (dont remember the correct order) ? But I proved the result in iii using induction of course

I'm not completely sure, the question says that there will be two values but I reckon you have to use induction to show what these cases are, i.e. to show that one case is n is even, resulting in -1; and n is odd, resulting in 1. It's kind of obvious that these will be the cases but I don't think you can assume it after trying a few cases

Reply 102

BenD0G

Hey guys, found this paper horrid, personally.

Anyway:
Q1: Did first part, then made correct subsitution of z=(1/y)(dy/dx) but didn't finish.
Q2: Complete.
Q5: Did all but the x^2-y^2 bit...but found IRRATIONAL points instead of rational ones > :frown:

Q6: Did it all but really unsure about final locus.
Q7: All but final summation result.
Q8: Complete.
Q10: Complete.

Any chance this could get me an S, ya think? :/

Reply 103

Original post by LucasJ94

I don't like repeating myself, especially on forums. But I had another night of bad sleep due to this, it's eating up my time even now when I'm finished with it.

Can anyone give me some peace of mind? Whether it's a 3, 2 or 1, I just want to have a guide as to what I can expect so that hopefully I can stop caring until the day of results.I haven't had a chance to go through the questions properly, but I'd guess you'll score in the mid-50s, which is probably a 2.

Reply 104

Original post by fruktas

In part ii, I am wondering, since the question said 'Find the values of.... in the two cases that arise., is it enough for state that it is 1/-1 for odd/even n (dont remember the correct order) ? But I proved the result in iii using induction of course

Since it only says "find", I think it's arguable that would be OK, but, for part (iii) it definitely says prove, so if you want to use the result from (ii) when doing (iii), you'd have to have proved it.

Reply 105

MrDD

email from the Cambridge Assessment people :

We very much regret the printing error in the STEP Mathematics Paper III taken on 27 June, which meant that candidates were unable to answer one of the (optional) questions. We are taking urgent steps to identify the candidates who may have been affected and to ensure that this does not affect their applications for university places. We have started a full review of our processes to make sure that errors of this kind are not repeated.
best wishes,
Admissions Testing Service
Cambridge Assessment

Reply 106

MrDD

Original post by DFranklin

Since it only says "find", I think it's arguable that would be OK, but, for part (iii) it definitely says prove, so if you want to use the result from (ii) when doing (iii), you'd have to have proved it.

yes, i think you may be right, although I don't think you need induction.

If you replace 0,1,2,3 in your working for part (i) with n,n+1,n+2,n+3 you can prove that there are 2 cases in part (ii) directly, and then substitute n=0, n=1 to find what they are for odd and even n, which then avoids the need for induction in part (iii)

Reply 107

Slowbro93

OP

20

Original post by MrDD

email from the Cambridge Assessment people :

We very much regret the printing error in the STEP Mathematics Paper III taken on 27 June, which meant that candidates were unable to answer one of the (optional) questions. We are taking urgent steps to identify the candidates who may have been affected and to ensure that this does not affect their applications for university places. We have started a full review of our processes to make sure that errors of this kind are not repeated.
best wishes,
Admissions Testing Service
Cambridge Assessment

I'm guessing all marks for efforts on q1?

Reply 108

Groat

13

Original post by cpdavis

I'm guessing all marks for efforts on q1?

Surely not!

Reply 109

MrDD

Original post by BenD0G

Hey guys, found this paper horrid, personally.

Anyway:
Q1: Did first part, then made correct subsitution of z=(1/y)(dy/dx) but didn't finish.
Q2: Complete.
Q5: Did all but the x^2-y^2 bit...but found IRRATIONAL points instead of rational ones > :frown:

Q6: Did it all but really unsure about final locus.
Q7: All but final summation result.
Q8: Complete.
Q10: Complete.

Any chance this could get me an S, ya think? :/

yes, i think there's a good chance. it depends how they are going to address the Q1 mess, but i think they will probably have an amended mark scheme for it that will be very generous to anybody who made any sensible progress.

Reply 110

Original post by cpdavis

I'm guessing all marks for efforts on q1?

Given the nature of the mistake, I don't think this would be a fair solution. It may be what they end up doing nonetheless, of course.

It's very very hard to undo the can of worms once you've opened it - I don't think there's a particularly equitable solution here.

Reply 111

Slowbro93

OP

20

Original post by Groat

Surely not!

Original post by DFranklin

Given the nature of the mistake, I don't think this would be a fair solution. It may be what they end up doing nonetheless, of course.

It's very very hard to undo the can of worms once you've opened it - I don't think there's a particularly equitable solution here.

What I meant is say that the candidate did a good attempt, then they can't be penalised to harshly. I think that's what they did for a D1 paper a couple of years back.

They could talk about lowering boundaries, but if it's because of that mistake then the argument is that you can get full marks without answering it.

This will be tricky. Question is how was it not noticed before dispatch? Surely they do a clerical check? :s-smilie:

Reply 112

It's an appalling mistake not to have noticed IMHO, it's pretty glaring, and it's the first question too!

Problem is that because it's the first bit, a lot of people will just have looked, thought "I don't get it", and done very little to no working. How you tell whether that means "spent 20 seconds on it", "spent 20 minutes on it", "didn't matter because they had lots of other questions they could do", "lost them 20 marks because they were banking on the differential equations question", etc. is gonna be hard.

[You also have people who "assumed" particular values for z. I don't see how you can dock marks for this under the circumstances].

Reply 113

Alexxh

But actually there was no more information needed, was there?
In my opinion, the sentence which was missing wasn't substantial.
There wasn't any need to assume how z looks like etc.

http://www.thestudentroom.co.uk/showpost.php?p=38365041&postcount=80
Or is that somehow wrong?

Reply 114

BabyMaths

Q4 part i

\displaystyle \sum_{n=1}^{\infty} \frac{1}{n!}=e-1

\displaystyle \sum_{n=1}^{\infty} \frac{n}{n!}=\sum_{n=1}^{\infty} \frac{1}{(n-1)!}=\sum_{n=0}^{\infty} \frac{1}{n!}=e

\displaystyle \sum_{n=1}^{\infty} \frac{n^2}{n!}=\sum_{n=1}^{\infty} \frac{n}{(n-1)!}=\sum_{n=0}^{\infty} \frac{n+1}{n!}=\sum_{n=0}^{\infty} \frac{n}{n!}+e=2e

\displaystyle \sum_{n=1}^{\infty} \frac{n^3}{n!}=\sum_{n=1}^{\infty} \frac{n^2}{(n-1)!}=\sum_{n=0}^{\infty} \frac{(n+1)^2}{n!}=1+\sum_{n=1}^{\infty} \frac{(n+1)^2}{n!}=5e

All the required results can be put together from these pieces. The final result being

\displaystyle \sum_{n=1}^{\infty} \frac{(2n-1)^3}{n!}=21e+1

Reply 115

jack.hadamard

4

Original post by DFranklin

It's an appalling mistake not to have noticed IMHO, it's pretty glaring, and it's the first question too!

After the exam, I kindly asked a person, who's been an examiner for years, to have a look at the paper and tell me what they think.
To be honest, they could not spot in the first minutes what we were actually supposed to do in this question (although in a meeting, but still).
In addition, to them it was strange that there is no evident space on the sheet to add much about

z

, and that it was definitely checked by someone.

In my opinion, everyone who has attempted successfully/unsuccessfully the first part should get 10 marks.
It is a really nice question, but it was terribly presented, and the student cannot be blamed for not "spotting" it.

Reply 116

Rahul.S

14

Original post by fruktas

AEA won't matter if Cambridge accept me :tongue:

I so hope I have done enough! Safmaster did very similar to me :smile:

your offer is 1,2 in any. Im quite sure uve got it. :smile:

Reply 117

Original post by Alexxh

But actually there was no more information needed, was there?

Well, yes there was, because

\displaystyle \dfrac{dz}{dx} = y^{n-1} \dfrac{dy}{dx}\left(n\left( \dfrac{dy}{dx}\right)^2 + 2y \dfrac{d^2y}{dx^2}\right)

is not a result, and so to write "Use the above result ..." was nonsensical.

In my opinion, the sentence which was missing wasn't substantial.

The number of upset students (and the response from the examiners)

There wasn't any need to assume how z looks like etc.

http://www.thestudentroom.co.uk/showpost.php?p=38365041&postcount=80
Or is that somehow wrong?

I don't see anything wrong with your solution, but I don't think that's the point in terms of the consequences of the misprint.

Edit: it's interesting that if they had written "use the above substitution ..." there would probably have been a lot less confusion. One word can make a lot of difference.

(edited 11 years ago)

Reply 118

B Jack