# STEP maths I, II, III 1991 solutions

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(Updated as far as post #191.) SimonM - 27.05.2009

Solutions for earlier STEP papers are not available on the internet, so we're making our own. Please submit any solution to any problem which is currently unsolved (red above); if you see any mistakes in solutions posted, please point them out.

(Several of the other threads still have one or two unsolved questions too - links are at the bottom of this post.)

N.B. the "usual" past paper website may have the papers mislabelled; please check the

1: Solution by justinsh and Dystopia

2: Solution in Advanced Problems in Mathematics (Siklos booklet I)

3: Solution by DFranklin

4: Solution by justinsh

5: Solution by generalebriety

6: Solution by nota bene

7: Solution by brianeverit

8: Solution by justinsh

9: Solution by brianeverit

10: Solution by brianeverit

11: Solution by brianeverit

12: Solution by brianeverit

13: Solution by brianeverit

14: Solution by brianeverit

15: Solution by brianeverit

16: Solution by brianeverit

1: Solution by Decota

2: Solution by Rabite and DFranklin

3: Solution by justinsh (alternative by khaixiang)

4: Solution by Rabite

5: Solution by Rabite

6: Solution by Glutamic Acid

7: Solution by Rabite

8: Solution by Rabite

9: Solution by brianeverit

10: Solution by brianeverit

11: Solution by Glutamic Acid

12: Solution by brianeverit

13: Solution by brianeverit

14: Solution by brianeverit

15: Solution by toasted-lion

16: Solution by brianeverit

1: Solution by generalebriety (alternative to (b) by justinsh)

2: Solution by generalebriety

3: Solution by Rabite

4: Solution by mikelbird

5: Solution by Speleo

6: Solution by Rabite

7: Solution by Rabite

8: Solution by squeezebox

9: Solution by mikelbird

10: Solution by brianeverit

11: Solution by brianeverit

12: Solution by brianeverit

13: Solution by brianeverit

14: Solution by brianeverit

15:

16: Solution by toasted-lion

1987 - 1988 - 1989 - 1990 - 1991 - 1992 - 1993 - 1994 - 1995 - 1996 - 1997 - 1998 - 1999 - 2000 - 2001 - 2002 - 2003 - 2004 - 2005 - 2006 - 2007

Solutions for earlier STEP papers are not available on the internet, so we're making our own. Please submit any solution to any problem which is currently unsolved (red above); if you see any mistakes in solutions posted, please point them out.

(Several of the other threads still have one or two unsolved questions too - links are at the bottom of this post.)

N.B. the "usual" past paper website may have the papers mislabelled; please check the

**front of the paper**, not the filename, to see which paper you're doing. (Mathematics = I, further mathematics A = II, further mathematics B = III.)**STEP I (Mathematics)**:1: Solution by justinsh and Dystopia

2: Solution in Advanced Problems in Mathematics (Siklos booklet I)

3: Solution by DFranklin

4: Solution by justinsh

5: Solution by generalebriety

6: Solution by nota bene

7: Solution by brianeverit

8: Solution by justinsh

9: Solution by brianeverit

10: Solution by brianeverit

11: Solution by brianeverit

12: Solution by brianeverit

13: Solution by brianeverit

14: Solution by brianeverit

15: Solution by brianeverit

16: Solution by brianeverit

**STEP II (Further Maths Paper A):**1: Solution by Decota

2: Solution by Rabite and DFranklin

3: Solution by justinsh (alternative by khaixiang)

4: Solution by Rabite

5: Solution by Rabite

6: Solution by Glutamic Acid

7: Solution by Rabite

8: Solution by Rabite

9: Solution by brianeverit

10: Solution by brianeverit

11: Solution by Glutamic Acid

12: Solution by brianeverit

13: Solution by brianeverit

14: Solution by brianeverit

15: Solution by toasted-lion

16: Solution by brianeverit

**STEP III (Further Mathematics Paper B):**1: Solution by generalebriety (alternative to (b) by justinsh)

2: Solution by generalebriety

3: Solution by Rabite

4: Solution by mikelbird

5: Solution by Speleo

6: Solution by Rabite

7: Solution by Rabite

8: Solution by squeezebox

9: Solution by mikelbird

10: Solution by brianeverit

11: Solution by brianeverit

12: Solution by brianeverit

13: Solution by brianeverit

14: Solution by brianeverit

15:

16: Solution by toasted-lion

**Solutions written by TSR members:**1987 - 1988 - 1989 - 1990 - 1991 - 1992 - 1993 - 1994 - 1995 - 1996 - 1997 - 1998 - 1999 - 2000 - 2001 - 2002 - 2003 - 2004 - 2005 - 2006 - 2007

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#2

I/2, II/5, II/6 can be found in Siklos' Advanced Problems in Mathematics, I will gladly scan them in here since this booklet is not available for free, but it's 7am here, I need to get some sleep so I will do it tomorrow

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#3

I'm attempting I/3 I'll see if I sort it out...

Okay, I'll post what I have now (probably should scan a sketch tomorrow as well), but it is not finished, and likely contains a few mistakes

I'm tired of this one, I'll take it up tomorrow again and try to sort it out.

Okay, I'll post what I have now (probably should scan a sketch tomorrow as well), but it is not finished, and likely contains a few mistakes

Spoiler:

Some observations I made was that if we let the first complex number have a length a the second will have a+ad etc. as long as we are in the first quadrant. This gives This forms a G.P. .

Considering the other quadrants it seems to hold under vector addition, however I'm a bit worried when I have a vertical vector for ad^{n-1}

By looking at this G.P. and how the numbers behave graphically it is evident that d<1 will mean it is a finite shape ending in a point (as ) and that d>1 will lead to an infinite shape (as . This is because we can see that the value of the angle theta is irrelevant as long as it is not zero or a right angle (vector addition will not work there).

Now to the question, expressing the complex number as . This leads us to . Therefore we can conclude that for all z the expression will represent d.

If we have a=1 so it nicely vanishes.

(I am insecure here if I need an explicit formula valid for all z, or just a recurrence relation, but I assume they want an explicit one)

As for the very last part it is easy to verify by a graph, but probably we need an algebraic formula instead, involving theta. I think this works: , where r_n is determined by the G.P. described earlier i.e. .

(As I am going along I realise I shouldn't be doing this straight into latex! Everything comes in a strange order, this formula would probably be good to mention earlier.)

When we have and as n gets large so does r_n and as the trig functions are periodic we will have an infinite structure geometrically.

Show

Some observations I made was that if we let the first complex number have a length a the second will have a+ad etc. as long as we are in the first quadrant. This gives This forms a G.P. .

Considering the other quadrants it seems to hold under vector addition, however I'm a bit worried when I have a vertical vector for ad^{n-1}

By looking at this G.P. and how the numbers behave graphically it is evident that d<1 will mean it is a finite shape ending in a point (as ) and that d>1 will lead to an infinite shape (as . This is because we can see that the value of the angle theta is irrelevant as long as it is not zero or a right angle (vector addition will not work there).

Now to the question, expressing the complex number as . This leads us to . Therefore we can conclude that for all z the expression will represent d.

If we have a=1 so it nicely vanishes.

(I am insecure here if I need an explicit formula valid for all z, or just a recurrence relation, but I assume they want an explicit one)

As for the very last part it is easy to verify by a graph, but probably we need an algebraic formula instead, involving theta. I think this works: , where r_n is determined by the G.P. described earlier i.e. .

(As I am going along I realise I shouldn't be doing this straight into latex! Everything comes in a strange order, this formula would probably be good to mention earlier.)

When we have and as n gets large so does r_n and as the trig functions are periodic we will have an infinite structure geometrically.

I'm tired of this one, I'll take it up tomorrow again and try to sort it out.

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#4

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#5

It's yours if you want it, I'm not going to be able to contribute properly for a day or two. I'll finish it if it's still left after that.

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#6

(Original post by

It's yours if you want it, I'm not going to be able to contribute properly for a day or two. I'll finish it if it's still left after that.

**Speleo**)It's yours if you want it, I'm not going to be able to contribute properly for a day or two. I'll finish it if it's still left after that.

III/1:

(a)

(b)

(c)

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#7

(Original post by

Would appreciate if someone could check (b) for me, I'm not sure it's right...

**generalebriety**)Would appreciate if someone could check (b) for me, I'm not sure it's right...

I think the (4r-1)x^(4r) seems wrong I'm trying to work out what it should be...

edit: having looked at the first four terms where both series get involved I'm inclined to believe it shall plainly be

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#8

(Original post by

I'm working it through now; firstly, where did you get the 1's from? surely they shouldn't be there? (or am I being stupid? anyway those doesn't affect the result)

I think the (4r-1)x^(4r) seems wrong I'm trying to work out what it should be...

edit: having looked at the first four terms where both series get involved I'm inclined to believe it shall plainly be

**nota bene**)I'm working it through now; firstly, where did you get the 1's from? surely they shouldn't be there? (or am I being stupid? anyway those doesn't affect the result)

I think the (4r-1)x^(4r) seems wrong I'm trying to work out what it should be...

edit: having looked at the first four terms where both series get involved I'm inclined to believe it shall plainly be

I think you're right about the other bit too, but I shall mostly take it on trust... need to do some French literature work, I shouldn't be doing maths.

Cheers.

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#9

(Original post by

I think you're right about the other bit too, but I shall mostly take it on trust... need to do some French literature work, I shouldn't be doing maths.

**generalebriety**)I think you're right about the other bit too, but I shall mostly take it on trust... need to do some French literature work, I shouldn't be doing maths.

About the question, I looked at more terms and as the difference between the two series always is a multiple of 4 I'm quite 100% sure I am right about that 3 now.

In fact I think all questions on power series, maclaurin and taylor seem very accessible and fairly easy on these old papers (what the hell

**I**'ve been able to do them!).

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#10

(Original post by

Hehe I recognise doing maths instead of what we are supposed to do .

About the question, I looked at more terms and as the difference between the two series always is a multiple of 4 I'm quite 100% sure I am right about that 3 now.

In fact I think all questions on power series, maclaurin and taylor seem very accessible and fairly easy on these old papers (what the hell

**nota bene**)Hehe I recognise doing maths instead of what we are supposed to do .

About the question, I looked at more terms and as the difference between the two series always is a multiple of 4 I'm quite 100% sure I am right about that 3 now.

In fact I think all questions on power series, maclaurin and taylor seem very accessible and fairly easy on these old papers (what the hell

**I**'ve been able to do them!).
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#11

**nota bene**)

I'm working it through now; firstly, where did you get the 1's from? surely they shouldn't be there? (or am I being stupid? anyway those doesn't affect the result)

I think the (4r-1)x^(4r) seems wrong I'm trying to work out what it should be...

edit: having looked at the first four terms where both series get involved I'm inclined to believe it shall plainly be

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#12

(Original post by

The 2nd line of your answer to (b) looks crazy; did you just forget to put in all the denominators in the log expansions? (i.e. a LaTeX error rather than a maths one).

**DFranklin**)The 2nd line of your answer to (b) looks crazy; did you just forget to put in all the denominators in the log expansions? (i.e. a LaTeX error rather than a maths one).

*me*there?

I assume you didn't, and yes there should be denominators in general's expression I think someone was lazy latexing.

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#13

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#14

**DFranklin**)

The 2nd line of your answer to (b) looks crazy; did you just forget to put in all the denominators in the log expansions? (i.e. a LaTeX error rather than a maths one).

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#15

(Original post by

Did you mean to quote

I assume you didn't, and yes there should be denominators in general's expression I think someone was lazy latexing.

**nota bene**)Did you mean to quote

*me*there?I assume you didn't, and yes there should be denominators in general's expression I think someone was lazy latexing.

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#16

Typing up I/6

i)

gives

Since the denominator is non-negative, for the derivative to =0 the numerator must be zero. Factorizing renders x=0 is a trivial solution, putting u=x^2 we can solve the quadratic that is left, giving we can reject -4 as that gives imaginary solutions for x and we are left with x^2=1 i.e. . This agrees with the reasoning in the question.

"Hence the stationary values are given by x=0, g(x)=3/4 and , g(x)=1" True.

"Since 3/4<1, there is a maxima at and a minimum at x=0"

The statement happens to be true, as we shall notice, but the argument is invalid as we cannot know if any of the points is a point of inflection. Therefore we test the points by looking at the value of the second derivative.

At x=0 g''(x)>0 which indicates a minimum, and at both x=1 and x=-1 g''(x)<0 which indicates a maximum.

"Thus we must have for all x"

This is not true as we know nothing of what happens when . We can easily prove the statement wrong by e.g. considering x=5 which gives g(x)=0.08...

ii) Their first statement that is false as they are differentiating (missing out a sign change as well) instead of integrating. It shall be:

Evaluating the integral means we have

as proposed (but with faulty argument as they differentiated...)

i)

gives

Since the denominator is non-negative, for the derivative to =0 the numerator must be zero. Factorizing renders x=0 is a trivial solution, putting u=x^2 we can solve the quadratic that is left, giving we can reject -4 as that gives imaginary solutions for x and we are left with x^2=1 i.e. . This agrees with the reasoning in the question.

"Hence the stationary values are given by x=0, g(x)=3/4 and , g(x)=1" True.

"Since 3/4<1, there is a maxima at and a minimum at x=0"

The statement happens to be true, as we shall notice, but the argument is invalid as we cannot know if any of the points is a point of inflection. Therefore we test the points by looking at the value of the second derivative.

At x=0 g''(x)>0 which indicates a minimum, and at both x=1 and x=-1 g''(x)<0 which indicates a maximum.

"Thus we must have for all x"

This is not true as we know nothing of what happens when . We can easily prove the statement wrong by e.g. considering x=5 which gives g(x)=0.08...

ii) Their first statement that is false as they are differentiating (missing out a sign change as well) instead of integrating. It shall be:

Evaluating the integral means we have

as proposed (but with faulty argument as they differentiated...)

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#17

(Original post by

No, I just can't do the Maclaurin series. At all. I always make stupid mistakes and I know none of them off the top of my head.

**generalebriety**)No, I just can't do the Maclaurin series. At all. I always make stupid mistakes and I know none of them off the top of my head.

I'm probably missing something obvious, must be the lack of sleep...

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#18

(Original post by

Well, what is the problem? I got the same answer as you (when including the fractions half-way).

I'm probably missing something obvious, must be the lack of sleep...

**nota bene**)Well, what is the problem? I got the same answer as you (when including the fractions half-way).

I'm probably missing something obvious, must be the lack of sleep...

There's no problem. I just do stupid things like ln(1+x) = 1 when x=0. Or I differentiate wrongly (especially with complicated functions). Or my arithmetic goes haywire. And I can't remember any of them off the top of my head.

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#19

(Original post by

Um, because I copied your answer without thinking when you corrected me. Because I hate the Maclaurin series.

There's no problem. I just do stupid things like ln(1+x) = 1 when x=0. Or I differentiate wrongly (especially with complicated functions). Or my arithmetic goes haywire. And I can't remember any of them off the top of my head.

**generalebriety**)Um, because I copied your answer without thinking when you corrected me. Because I hate the Maclaurin series.

There's no problem. I just do stupid things like ln(1+x) = 1 when x=0. Or I differentiate wrongly (especially with complicated functions). Or my arithmetic goes haywire. And I can't remember any of them off the top of my head.

ugh, I'm bored of latexing, but I'll finish that question I'm doing now and then try to solve some more...

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#20

**nota bene**)

Did you mean to quote

*me*there?

I assume you didn't, and yes there should be denominators in general's expression I think someone was lazy latexing.

Sorry again...

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