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#1 Re: Help Me ! » Optimization » 2010-02-05 06:55:11

Thank you so much for replying. Can you please elaborate on how you got that as a result? (i.e. how you solved part 1, 2 and 3)

#2 Help Me ! » Optimization » 2010-02-04 08:10:14

Hioj
Replies: 8

NB: This is translated from a different language to English, but I hope you will be able to understand it anyway.

Assignment
A fence must be put up by a house as shown in the attached figure. The house is the hatched and blue part, and the fence is indicated by the broken lines. In total 60 m fence must be put up (i.e. |AB|+|BC+|CD|+|DE|=60), and all angles in the fence are right angles.

The length of line AB is designated x.
1)     Determine the lengths |CD|, |BC| and |DE| expressed by x.
2)     Determine the area of the fenced part, expressed by x.
3)     Determine the dimensions of the fence, so that the area of the fenced part become as big as possible.



Solution
1) It's obvious that |CD| is

. I can't figure out how to find |BC| and |ED| expressed by x. So this is where I get lost.
2) Expressing the area by x is
.
3) How do I make a function to describe this?


Thanks in advance!


Figure:
optimizationassignment.png

#3 Re: Help Me ! » Math competitions » 2009-11-30 09:21:30

Thank you so much!

Do you know other pages with previous math competition sets? I've only been able to find this one page.

Would you mind if I kept posting in this page with my solutions to the different problems and asking questions if I can't solve it?

#4 Help Me ! » Math competitions » 2009-11-29 10:03:02

Hioj
Replies: 5

Hello everyone.

I'm entering math competitions now and I'm going to be training for them. What I need to train are proofs. To give you an idea of how the problems are, here are some links to the previous years' competitions:

2009 - http://www.georgmohr.dk/gmopg/gm09pb.pdf

2008 - http://www.georgmohr.dk/gmopg/gm08pb.pdf

2007 - http://www.georgmohr.dk/gmopg/gm07pb.pdf

Can you give me any guidance?

Thanks!

#5 Re: Help Me ! » A project in probability » 2009-11-01 08:31:53

That is very interesting! I've never heard about a tree structure before in mathematics, but I'll read up on it. Thanks for bringing such awesome solutions!

#6 Re: Help Me ! » A project in probability » 2009-10-31 12:35:28

By the way, what software are you using to do that, bobbym?

#7 Re: Help Me ! » A project in probability » 2009-10-31 11:07:20

Okay I did question c)... took quite some time, but here are my results:

    Team A wins:  0.18873
    Team B wins:  0.56057
    Tie: 0.2507

They are similar to the ones from your calculation, so I suppose they are correct! smile

Thank you so much for your help. I really appreciate it !!!

#8 Re: Help Me ! » A project in probability » 2009-10-31 09:54:01

Thanks maths! I get it now. I'll report back on my results later today or tomorrow.

Wow, bobbym, amazing. Thank you so much. How did you run that computer simulation? smile

#9 Re: Help Me ! » A project in probability » 2009-10-31 07:44:56

Thanks so much for your answer! That is really helpful (question b involved the 2nd last shot from team B, you've done it as if it was their 3rd last shot, but nevermind--your example described the method perfectly). I have a few questions:

In your example, how do you find the probability to be 0.09 for team B scoring 3, 4 and 5 goals? Shouldn't their respective probability for scoring be 0.7, since all the events are independent? Thus for the 3rd goal the probability would be 0.7, 4th goal 0.49 and 5th goal 0.343.

Why does team A have a 0.3 chance of scoring in their 4th shot?


Say team B has to shoot for the 2nd last time, how does one fill out your "combined probabilities table"? I can't get a probability of team B winning, although there must be one: Team B scores on its last 2 shots, team A misses its last one.

#11 Re: Help Me ! » A project in probability » 2009-10-30 07:58:06

Exactly! That's exactly what I'm looking for! Would you mind explaining your notation in the B wins and A wins?

Are you sure you can say 0.3^9 for the tie? A tie doesn't necessarily mean 10 misses, just that they each have scored the same number of goals: if team A has scored 3 and team B 3 as well, the match ends with a tie.

#12 Re: Help Me ! » A project in probability » 2009-10-30 06:45:27

Thanks for trying to help me out! I'm sorry if my description was unclear. I'm from Denmark and thus I'm not sure about the correct terms used in a soccer match, but I'll try once more:

The problem is about the outcome of a soccer match. None of the teams manage to score so they get to the part of the game where each team have to shoot 5 times. They alternate, i.e. team A shoots, then team B, then team A etc. until they each have shot 5 times.

In most cases there's a probability that either team A or team B wins or the match ends equal. We have to figure these probabilities out in the following cases:

a) None has missed and team B has to shoot for the last time.
This means that team A has scored 5 times, team B has scored 4 times, so what's the probability that team A will win, that team B will win (won't happen in this case) or that the match ends equal?

b) None has missed and team B has to shoot for the 2nd last time.
Team A has scored 4 times, team B has scored 3 times, now what's the probability that team A will win, that team B will win or that the match ends equal?

c) A has missed the first shot.
A has shot only once, and missed it. Team B hasn't shot yet - 5 shots left. What's the probability that team A will win even though they missed their first shot, what's the probability that team B will win and that the match will end equal?

d) Find own cases of interest.



I hope that clarifies some things for you smile

#13 Help Me ! » A project in probability » 2009-10-29 06:31:35

Hioj
Replies: 19

Hello everyone. We have a problem in school with a lot of different probability problems, including the following, which I'm not 100% sure about:

Penalties

A match between team A and team B has to be decided on penalties. Every team has to kick 5 times. We assume that be every try there's a 70% chance of making a goal. Team A starts. Find the probability that after 5 tries to each team, team A has won, team B has won or the penalties end equal, when:
a) No team has missed and team B has to kick for the last time.
b) No team has missed and team B has to shoot for the 2nd last time.
c) A has missed the first shot.
d) Find more of interest on your own.



The way I think about it is this:

Definition:
S = Score
F = Fail

a) team A has a 30% chance of winning and thus team B has a 30% chance of losing. There's a 70% chance of the penalties ending equal.

b) There are several ways the match can end:
Team A score is: SSSS-
Team B score is: SSS--

Assuming team A wins:
B: 4. S 5. F
A:        5. S

B: 4. F (5. S or 5. F - doesn't matter)
A:         5. S

So the probability of team A winning would be the probability of each of these, i.e.

and

and


Assuming team B wins:
B: 4. S 5. S
A:        5. F

Probability:


Assuming the match ends equal:
B: 4. S 5. S
A:        5. S

B: 4. S 5. F
A:        5. F

Probabilities:

and


c)
There are LOTS of combinations here:

Assuming team A wins:
1.
A: 1. M 2. S 3. S 4. S 5. S - 4 scores
B: 1. M 2. M 3. S 4. S 5. S - 3 scores 2 misses

Team B is a binomial, team A is not:

2.
A: 1. M 2. M 3. S 4. S 5. S - 3 scores 1 miss
B: 1. M 2. M 3. M 4. S 5. S - 2 scores 3 misses

Both teams are binomials:

...and so forth.


Assuming team B wins:
1.
A: 1. M 2. S 3. S 4. S 5. S
B: 1. S 2. S 3. S 4. S 5. S

...continues as above.


Equal:
The same.


d)
Both teams have shot twice, but team A has missed both of their kicks. What's the probability team A will win?

and

Team A has shot three times and missed one. Team B has missed all three. What's the probability the match will end equal?

#14 Re: Help Me ! » Probability when choosing cards » 2009-10-25 08:42:55

Thanks guys!
I knew it was a binomial experiment, I just couldn't see it had to be (12 9)... I was trying with (52 12) but the answer seemed way off. Thanks again!

#15 Help Me ! » Probability when choosing cards » 2009-10-25 07:42:05

Hioj
Replies: 3

The problem is as follows:
"A card is drawn from a deck of cards, the card is noted and thereafter returned to the deck. This experiment is repeated 12 times. Find the probability for the following events:
A: There are exactly 9 court cards"

There are 3 more events but more or less the same as A, so I omitted them. The formulation of the problem is originally also not in english, I've translated it.

This is what I've got so far:

Total court cards per deck: 12 <=> probability of choosing a court card is 12/52.
Thus the probability for choosing a court card 9 times must be (12/52)^9.

I'm unsure about what to do next. Should I multiply it by (40/52)^3 since that's the probability of the 3 remaining picks from the deck?


Thanks for helping!

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