Another way to answer the second part is as follows(Original post by maltodextrin)
I'm probably wrong but to show that the integral lies between 0 and 1 would it not be sufficient to say that
(x^2)(e^x)/(x + 1) < (x^2)(e^x)/x = x(e^x) for 0 < x < infinity
then integrate x(e^x), which equals 1. Seems a lot simpler?
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STEP Maths I, II, III 1988 solutions
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 19062011 17:39

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 82
 25062011 17:06
(Original post by SimonM)
....
Using the formula for arc length:
Total distance.
So, if
so, total distance,
.
To find the minimum we must differentiate but, before we do that, let us turn
into
where F is the antiderivative operator acting on .
So, differentiating w.r.t. b:
The derivative and antiderivative cancel giving:
.
To find stationary point, let the derivative be 0:
(Notice this is the only stationary point in the interval (0,a) which is the range we care about).
Now, to see whether this really is minimum, we must find the 2nd derivative:
.
Since gives the minimum the length of road.Last edited by Dirac Spinor; 26062011 at 23:24. 
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 83
 31072011 07:07
(Original post by bensmith)
STEP III Q4
Using the formula for arc length:
Total distance.
So, if
so, total distance,
.
To find the minimum we must differentiate but, before we do that, let us turn
into
where F is the antiderivative operator acting on .
So, differentiating w.r.t. b:
The derivative and antiderivative cancel giving:
.
To find stationary point, let the derivative be 0:
(Notice this is the only stationary point in the interval (0,a) which is the range we care about).
Now, to see whether this really is minimum, we must find the 2nd derivative:
.
Since gives the minimum the length of road. 
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 84
 31072011 10:39
(Original post by brianeverit)
1988 STEP III number 3.
I'm afraid I disagree with the solutions published so far. Here is my solution.

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 85
 31072011 10:44
(Original post by mikelbird)
I think that the three parts to this question are entirely separate and do not include the condition of being on the original circle...that is where we disagree. As regards the other question (4) can you see where we differ on the diffentiation...that is where the theorem comes in.... 
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 86
 31072011 16:34
yup!!

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 87
 30082011 01:02
(Original post by Glutamic Acid)
I/7.
Therefore
If f(x) <= 1, then 1 <= f(x) <= 1.
If f'(x) <= 4, then 4 <= f(x) <= 4.
f'(x) = f(1)(x + 1/2) + f(1)(x  1/2)  2f(0)x.
The largest values of f'(x) will be when x = 1 or 1 to maximize the values of (x + 1/2) and (x1/2). The largest value of f(1)(x + 1/2) can be 3/2 when f(1) = 1. The largest value of f(1)(x  1/2) can be 1/2 when f(1) = 1. The largest value of 2f(0)x can be 2 when f(0) = 1. 3/2 + 1/2 + 2 = 4, so f'(x) <= is satisfied. When x = 1, the largest value can be (1)(1/2) + (1)(3/2)  (2)(1)(1) = 4.
The smallest values of f'(x) will be when x = 1 or 1 likewise. When x = 1, be (1)(3/2) + (1)(1/2)  2(1) with f(0) = 1, f(1) = 1 and f(1) = 1. This gives 4. When x = 1, it will be (1)(1/2) + (1)(3/2)  2(1) = 4, with f(1) = 1, f(1) = 1 and f(0) = 1, giving 4. Therefore f'(x) <= 4 is still satisfied.
Letting f(0) = 1, this gives c = 1. Letting f(1) = 1, this gives a + b  1 = 1 hence a + b = 2. Letting f(1) = 1, this gives a  b = 2. Hence 2a = 4, a = 2 and b = 0.
f(x) = 2x^2  1.
Sorry for any errors.
STEP I Q7:
Surely, the conditions in the question also allow f(x)= 2x^2 +1 for the last part? This should also be true simply by symmetry of the modulus signs and the fact that either f'(1)=4 or f'(1)=4.
I could of course be wrong, but someone please clarify. 
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 88
 31082011 19:57
Let be the velocities/speeds of ball 1 and 2 with u being before the collision and v after.
Momentum is conserved in a closed system so:
The coefficient of restitution is < 1 so kinetic energy is not conserved amongst the two balls, i.e:
Combining the two:
Where is the angle between the two velocities as desired.
For the next part, let be the angle between .Separating momentum into two components and and considering restitution in the direction of we obtain 3 equations, sufficient to eliminate the three speeds and obtain an equation in terms of our two angles, thus confirming the intuitive idea that the initial velocity of the first ball is irrelevant:
By adding the first and last and substituting, we obtain:
The angle of deflection is which we shall call k. Consider
Notice that the maximum of Tank will occur for the same value of theta as the maximum of k so, differentiating both sides w.r.t. theta we get:
Which, if we equate it to zero, gives us:
So:
as required.
Comment: I found this Q particularly difficult and it has taken me many attempts to get it right. Comments are welcome.Last edited by Dirac Spinor; 03092011 at 11:51. 
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 89
 02092011 18:45
i) If no pressure is exerted on the wire by the bead and (by N3) vice versa, then the wire might as well not be there i.e. the bead is projected under gravity alone and follows the path of the wire. The general equation of motion of a particle freely projected under gravity can be derived by considering the suvat equations in the x and y directions so, horizontally (note that the derivative of the equation of the wire at x=0 is one which means the angle of projection is pi/4:
Vertically:
Substituting to eliminate t:
We want this to be the same as the equation of the wire so:
ii) If is the angle that the velocity makes with the x axis then:
This also allows us to calculate the bead's velocity in terms of . Letting it's position be r:
and so, by conservation of energy:
Initial K.E.= K.E. + G.P.E
as required. 
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 90
 02092011 22:39
This distribution is hypergeometric and, whilst I quoted it seeming as it is not an uncommon distribution in STEP questions, it's PMF can be derived like this:
By symmetry, all configurations are equally likely so the probability that we pick a certain number of reds is the number of ways we can do this divided by the total number of configurations. Let this event be I:
i)Forgetting, for the moment, that there are colours, the probability that any particular ball is chosen at the ith selection is, by symmetry, the same i.e and so the probability that a red ball is drawn at the ith selection is the sum of the probabilities that each particular red ball is drawn on the ith go:
as required.
ii) In a similar way, we can see that the probability of one particular ball being chosen on one particular selection and similarly for another ball is but, if we take into account colour, we must times this by 2 as we need to consider the situation in which these two particular balls are swapped as we need not distinguish between any two red balls. Now that we have the probability of one particular pair of i and j values, we need to multiply this by the total number of pairs of reds:
as required.
To find the mean:
To find the variance, we must first find :
Now we can use part ii to evaluate the sum except we must be careful to take into account that it is only valid for . When i=j, this result becomes because the other automatically happens if one of the two happens. This second case happens n times out of the total of n^2 times so:
Simplifying:
Last edited by Dirac Spinor; 05092011 at 01:09. 
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 91
 19092011 00:40
(Original post by SimonM)
....
Throughout, if I don't put a vector in bold it is because I am referring to it's modulus. Also, when I refer to coordinates I am doing so relative to axes with origin the goal line and in the plane of the balls motion. +ve x is away from the goal and +ve y is away from the ground.
Let's find the resultant force on the ball. The force due to gravity will be and the force due to the wind is mk times the velocity of the ball relative to the wind velocity i.e. as required.
Now, this equation can separated into it's horizontal and vertical components as are perpendicular. Horizontally:
Now we can consider the initial conditions and :
Using the initial conditions in the same way:
. Now for the vertical component:
For the next part, note that if the ball returns to the origin for a second time, an own goal will be scored. So, consider the case x=y=0:
if y=0 then .
We already know about the t=0 solution,we are interested in the second root:
As required. 
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 92
 19092011 00:43
(Original post by SimonM)
.... 
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 93
 31012012 19:57
Here is my answer for Question 11, which curiously doesn't seem to have been done yet (?) for Paper 3 (i.e. 1988 Paper FM B Q11).
Just for novelty I've posted my answer as a youtube video, although warning it is 40+ minutes long and the tone will be a bit too didactic (i.e. boring) for most of the kind of people viewing a thread like this (It's primarily intended for students at my school).
Edit: For anyone just double checking their answer, I got (i) and (ii)Last edited by waxwing; 31012012 at 20:09. Reason: Add numerical answer 
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 94
 14032012 11:05
(Original post by waxwing)
Here is my answer for Question 11, which curiously doesn't seem to have been done yet (?) for Paper 3 (i.e. 1988 Paper FM B Q11).
Just for novelty I've posted my answer as a youtube video, although warning it is 40+ minutes long and the tone will be a bit too didactic (i.e. boring) for most of the kind of people viewing a thread like this (It's primarily intended for students at my school).
Edit: For anyone just double checking their answer, I got (i) and (ii) 
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 95
 14052012 20:02

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 96
 14052012 23:01
(Original post by kabbers)
III/1:
Sketch
I'm not going to bother posting most of the differentiating legwork, there arent too many tricks, its just an algebra bash.
Differentiating (quotient rule is your friend), we get
So we have turning points at x = 0, and x =
Differentiating again, we get
Substituting in values of x for the turning points, we find that x = 0 is a minimum, and the other two maximums.
Noticing the denominator of y, we find that there is an asymptote at x = 1.
And because of the exponential properties of e^x, y tends to zero as
y tends to as since the numerator remains positive and given the exponenential properties of e^x much greater than the denominator, while the denominator becomes negative.
So the graph will look http://www.thestudentroom.co.uk/show...8&postcount=14
Prove
First note that
Hence we may split the integral into
Consider
Now consider
I posit the inequality for x > 0
So our inequality holds.
So:
Thus
And therefore,
Now notice that the graph of is greater than 0 for x > 0, and hence so is the infinite integral.
So,
please point out any mistakes
Is this correct?

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 97
 14052012 23:25

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 98
 14052012 23:50
(Original post by kabbers)
relatively straightforward q, pointing out any mistakes will be appreciated as always
Ok first note that at the end of day n, the firm will owe everything it owed the previous day, the interest on this plus everything it has borrowed that day.
This may be expressed as
By evaluating the first few terms of this sequence, it becomes easy to 'guess' and then prove a total value for Sn:
ok my guess is that (by geometric series)
this fits the questions, but we have to prove it (induction!)
Basis case: , therefore the basis case works.
Inductive step:
This is in the same form as the above, so it works.
for part 2, firstly we can assert that (this is the initial amount owed)
the firm must pay interest on what it owes then pay back what it has earned.
Again, let's try a few values:
hypothesis:
(again by geometric)
we need to prove this, again by induction.
basis case: when m = 0, basis works
inductive step:
Hence the sum works. However the question did not ask for us to express S_T, so we must substitute in (see our above def. for S_T)
for part 3, note that if the sequence is decreasing, the firm will pay off its debt. so the conditions are:
Letting x = k+1
taking everything on to one side
k+1 > 0, so both x^m and (x1) are greater than zero. Therefore, for the inequality to be satisfied,
as required.
If being decreasing is sufficient for the sequence to become negative finally? Maybe there is a positive limit there for it to reach as it decreases. So far I have no idea how to prove that there is no such a limit. Hope that you could have a check on this if you have time. Thx! 
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 99
 17052012 20:01
btw, how do you find the value of x, y, z here? I did it by let y=z in the last equation so we get x and then substitute back to the first equations. Then get a quadratic equation of y. Two roots, 2/3a and 1/6a 
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 100
 31052012 11:37
[/I]
(Original post by mikelbird)
I get the same as you....see file...
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Updated: May 26, 2016
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