You are Here: Home >< Maths

The Proof is Trivial! Watch

1. I tend to avoid second order differential equations because they can become quite nasty. In this case solving the first order DE was somewhat neater and very STEP-like with the discriminant. Energies is nice too, although it's usually the last thing I think of.
2. (Original post by und)
Solution 50

Using as the initial velocity.

Since the function of is continuous, if for some , then at that instant the frictional force must be greater than the weight, thus the rope will remain stationery. It remains to find a range of values of for which will become .

Creating and solving the differential equation, we obtain , so if is to be for some positive , then there must exist a positive root , so a necessary and sufficient condition (assuming all the constant terms are positive) is as required.

If the rope has mass , then the impulse applied to accelerate the rope to a speed is given by the change of momentum . Hence , giving the maximum impulse as .
I think this is the most natural solution to the question. But it is very STEP-esque to have multiple possible solutions.
3. (Original post by und)
Sorry to be blunt but this is all rather unconvincing. Consider what happens if we decide to make theta smaller. We can then increase ab.
I said that if you increase the side length of 3, then the other side can becomes at most 3. This means that you have this new side of length 3 and the old one of length 2. If you increase the one of length 2 the other side must become at most 2, hence you still have sides of at most length 3 and 2. If you increase both then you have sides lengths of at most 2, just over 3 and just over 3. But you can have side lengths of 2 and 3 and one larger so this still gives the most area as the area depends only on the two sides 2 and 3 and the angle between them.

If it wasn't clear enough in my original post I am editing it now.

Edit: I'll admit that my original post didn't have the best explanation, but the idea was there that you couldn't get longer shortest side lengths than of 2 and 3.
4. Problem 56 **/***

Evaluate

5. (Original post by metaltron)
I said that if you increase the side length of 3, then the other side can becomes at most 3. This means that you have this new side of length 3 and the old one of length 2. If you increase the one of length 2 the other side must become at most 2, hence you still have sides of at most length 3 and 2. If you increase both then you have sides lengths of at most 2, just over 3 and just over 3. But you can have side lengths of 2 and 3 and one larger so this still gives the most area as the area depends only on the two sides 2 and 3 and the angle between them.

If it wasn't clear enough in my original post I am editing it now.

Edit: I'll admit that my original post didn't have the best explanation, but the idea was there that you couldn't get longer shortest side lengths than of 2 and 3.
I see what you mean, but it could have been explained better in the solution. So your argument is as follows if I understood correctly.

Spoiler:
Show
Label the three sides of the triangle , and and assume without loss of generality that . Then , and . Now consider the two shorter sides of the triangle, and . Label the angle between them , and consider the formula for the area of the triangle given by . Clearly this is a maximum when , meaning we have a right triangle. We check that the condition is satisfied when and take their respective maximum values, and it is because . Hence the maximum area is given by .

I'm very disappointed I never gave this question greater thought in the actual exam. This is a nice solution.
6. (Original post by Indeterminate)
Problem 56 ***

Evaluate

Isn't there a really easy way of doing this, only needing A-Level knowledge? (Don't read the spoiler if you want to do the question)

Spoiler:
Show
use the t sub?
7. (Original post by und)
I see what you mean, but it could have been explained better in the solution. So your argument is as follows if I understood correctly.

Spoiler:
Show
Label the three sides of the triangle , and and assume without loss of generality that . Then , and . Now consider the two shorter sides of the triangle, and . Label the angle between them , and consider the formula for the area of the triangle given by . Clearly this is a maximum when , meaning we have a right triangle. We check that the condition is satisfied, and it is because . Hence the maximum area is given by .

I'm very disappointed I never gave this question greater thought in the actual exam. This is a nice solution.
Yeah that's it! It's a really easy question if you can word your answer right, but for some reason in that exam I had so many mental blocks. Was pretty upset after that paper, especially since it was the easiest in ages. Anyway, bring on next year's!
8. (Original post by und)
I see what you mean, but it could have been explained better in the solution. So your argument is as follows if I understood correctly.

Spoiler:
Show
Label the three sides of the triangle , and and assume without loss of generality that . Then , and . Now consider the two shorter sides of the triangle, and . Label the angle between them , and consider the formula for the area of the triangle given by . Clearly this is a maximum when , meaning we have a right triangle. We check that the condition is satisfied, and it is because . Hence the maximum area is given by .

I'm very disappointed I never gave this question greater thought in the actual exam. This is a nice solution.
The better way of explaining this I think is once you get to the formula for the area, to say something like:

"We cannot use a side of length 4 as one of the legs of the triangle, as the hypotenuse would be larger than 4, which isn't allowed. Thus take the legs of the triangle to be 2 and 3; by Pythagoras, the hypotenuse is 3^2 + 2^2 = 13 < 4^2 (so satisfies the restrictions). Hence the maximum area of the triangle is (1/2)*2*3=3."

(This is obviously the same as what you've written, except I think you should make the point about the a leg of length 4 explicit.)

Are all BMO1 problems this short?
9. (Original post by shamika)
The better way of explaining this I think is once you get to the formula for the area, to say something like:

"We cannot use a side of length 4 as one of the legs of the triangle, as the hypotenuse would be larger than 4, which isn't allowed. Thus take the legs of the triangle to be 2 and 3; by Pythagoras, the hypotenuse is 3^2 + 2^2 = 13 < 4^2 (so satisfies the restrictions). Hence the maximum area of the triangle is (1/2)*2*3=3."

Are all BMO1 problems this short?
Isn't it OK simply to consider the two shorter sides which are less than 2 and 3 respectively?

This year the paper was much easier than usual. The only reasonably difficult question was Q6 - I wasted a lot of time trying to solve that one without any success.
10. Problem 57 *

find

11. (Original post by shamika)
The better way of explaining this I think is once you get to the formula for the area, to say something like:

"We cannot use a side of length 4 as one of the legs of the triangle, as the hypotenuse would be larger than 4, which isn't allowed. Thus take the legs of the triangle to be 2 and 3; by Pythagoras, the hypotenuse is 3^2 + 2^2 = 13 < 4^2 (so satisfies the restrictions). Hence the maximum area of the triangle is (1/2)*2*3=3."

(This is obviously the same as what you've written, except I think you should make the point about the a leg of length 4 explicit.)

Are all BMO1 problems this short?
The completing the square one was even shorter surely?
12. (Original post by shamika)
Isn't there a really easy way of doing this, only needing A-Level knowledge? (Don't read the spoiler if you want to do the question)

Spoiler:
Show
use the t sub?
Spoiler:
Show
The bit in bold misled me into NOT wanting to try Weierstrass!
13. (Almost complete) Solution 27

This representation is all well and good, but as noted by my earlier attempts, any attempt to evaluate in any coordinate system with the integrand in this form leads to a very cumbersome/impossible integral. The aim is thus to represent it in an equivalent form that will make the integration easier.

Notice that if are the vectors such that , then:

If we take the unit vector in the direction of and call it , then

. In order to be able to express any point in , we need two more unit vectors which are all perpendicular to each other, say . As the three vectors are mutually orthogonal and of unit length in their directions, then they can now form an alternative way of expressing points in and so form an orthonormal basis.

Let these three unit vectors have associated coordinates . Then since they are orthonormal, the region we are integrating over can still be written in the same form, just with the coordinates substituted for . In our new orthonormal basis:

and so we can rewrite as:

, with the region in that we are integrating over defined by . However, this could be potentially a troublesome integral because of the limits and so this integral would better be evaluated in the spherical coordinate system rather than the cartesian. So changing the variables over redefines the region in that we are integrating over as , multiplying by the Jacobian and picking our order of integrating carefully so as not to run into trouble gives:

One iteration of integration by parts gives

The second part of this question is still to be completed.
14. (Original post by DJMayes)
No, that's fair enough. I tend to opt for Differential Equations because I like transforming a Mechanics question into a Pure question (And one of the nicer Pure topics in my opinion) but some of the energy solutions are really nice.
Yeah - DEs can give really neat solutions for mechanics questions, e.g. SHM.
15. Solution 57

Let the integral be . Apply to get (by the oddity of tan), then add the two forms of together and divide by 2 to obtain .
16. (Original post by Farhan.Hanif93)
Solution 57

Let the integral be . Apply to get (by the oddity of tan), then add the two forms of together and divide by 2 to obtain .
Nice, my method was to split it into 2 integrals, 1/(1+x^2) and another where the integrand was odd so it went to 0.
17. (Original post by Farhan.Hanif93)
Solution 57

Let the integral be . Apply to get (by the oddity of tan), then add the two forms of together and divide by 2 to obtain .
Damn beat me to it :/
18. Well I guess I'll provide another physicsy mathsy problem

Problem 58 **/***

A double pendulum consists of a mass m2 suspended by a rod of length l2 from a mass m1, which is itself suspended by a rod of length l1 from a fixed pivot, as shown below.

Show that the equations of motion for small displacements can be written as

where,

and
19. Hint for 56:

Consider the integral

with the substitution

Residue theorem!
20. Solution 57
Spoiler:
Show

Note that is an odd function in the interval (as it is the product of the even function and the odd function ) and ergo

Therefore the answer is

Why I am so slow with latex T_T

I may give the residue theorem a go later

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: December 11, 2017
Today on TSR

Does race play a role in Oxbridge admissions?

Or does it play no part?

The worst opinion about food you'll ever hear

Discussions on TSR

• Latest
• See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• Poll
Useful resources

Make your revision easier

Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

How to use LaTex

Writing equations the easy way

Study habits of A* students

Top tips from students who have already aced their exams

Create your own Study Planner

Never miss a deadline again

Thinking about a maths degree?

Chat with other maths applicants

Can you help? Study help unanswered threads

Groups associated with this forum:

View associated groups
Discussions on TSR

• Latest
• See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.