Math Is Fun Forum
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#151 Re: Dark Discussions at Cafe Infinity » We missed it! » 2009-09-21 20:56:38
Hmm well maybe it didn't affect Australia cuz I didn't feel anything.
(Pictures Australia floating out in space by itself)
#152 Re: Puzzles and Games » Too tough for a homework assignment? » 2009-09-20 21:05:38
Hope what I've done is right... it is quite a tough homework assignment, but maybe because of the lengthy algebra
Oh dear Jane got it so quickly 
#153 Re: Help Me ! » Standard Deviation » 2009-09-12 22:27:52
The forum already has it installed if you want to use it, just put
tags around the latex
#154 Re: Exercises » Identity » 2009-09-12 05:11:15
Hi bobbym, is it meant to be:
#155 Re: Help Me ! » Standard Deviation » 2009-09-11 17:04:03
You can click on the symbols to find out how to write them in latex
is \bar{x}_n is s_n^2 is \frac{n}{n-1}thankyou
#156 Help Me ! » Standard Deviation » 2009-09-10 21:04:24
- Identity
- Replies: 5
Let
and denote the sample mean and variance for the sample , and le and denote these quantities when an additional observation, is added to the sample.Show that
a) Show how
b) Show that
I've done question a) but I'm stuck on question b) can anyone please help!
#157 Re: Dark Discussions at Cafe Infinity » Disturbing... » 2009-09-09 21:32:44
They were stuck, you explained, they probably learned something.
What if they asked their dad and he gave the answer. Is that cheating?
"Cheating" is answering lots of question for them, without explanations.
Does the same still apply to a serious highschool assignment though?
On another site I go to, lots of people post up their take-home 'School-assessed coursework', which comprise 10-15% of their mark for their VCE subject. They get their help and say that since they're learning, it's morally ok... and disturbingly the site seems to be 'ok' with it.
I just think it's horribly unfair on everyone else.
Where do you draw the line!?
#158 Re: Help Me ! » Finding the domain of a changed pdf » 2009-09-02 01:23:36
Haha, you're welcome!
It is curious that you calculated the area underneath f(y) correctly, and not for f(x). That for f(y) is much more difficult!
Jose
Yes, I tend to overlook simple things because I don't think they are worth my time. Perhaps I should learn better
#159 Re: Help Me ! » Finding the domain of a changed pdf » 2009-09-01 21:25:25
Oh I made another mistake with the area XD
But now I know how to find the domain
Thanks so much juriguen!
#160 Help Me ! » Finding the domain of a changed pdf » 2009-09-01 19:41:39
- Identity
- Replies: 5
Let X be a random variable with density function
Find the density fuction of
How can this be a density function if the area underneath it is not equal to 1?
Anyway... I was able to find the new density function,
, but I don't know how to logically find it's domain.The answer says the domain is
, I guess because the area under the new density function from 0 to 8 is equal to 1. But the original density function didn't have an area equal to 1 so why must this one
?Also, I think
is pretty arbitrary, how come you can't have something like ? That gives an area of 1 as well.#161 Re: Maths Is Fun - Suggestions and Comments » Images won't delete » 2009-09-01 18:50:43
Dear Mathsisfun, I've noticed that if I make a post, then for about 5 minutes after I post it (not exactly sure how long) I can't delete the post. I can click the button 'Delete', but it doesn't do anything. I have to wait a long time to come back and delete it.
#162 Re: Help Me ! » changing variables in a pdf » 2009-09-01 18:44:08
Thanks juiguen, I just realised I got the formula for integration wrong:
#163 Help Me ! » changing variables in a pdf » 2009-09-01 04:31:47
- Identity
- Replies: 3
There is the pdf
and
. You need to find .I used two methods but they come up with difference results:
1.
The cdf
So
Note that I used the formula
2.
Using the formula
#164 Help Me ! » Changing the variables of a pdf » 2009-08-31 05:04:21
- Identity
- Replies: 1
Say you have a pdf
and you have (for example) and you wanted to find , what does this change of variables actually achieve, and how are you meant to interpret the result? I've read over it but I don't really get the concept.#165 Re: Help Me ! » Double integral » 2009-08-30 23:24:58
Thanks everyone, in the end I think rzaidan's proof was easiest for me to understand.
#166 Help Me ! » Double integral » 2009-08-29 23:13:46
- Identity
- Replies: 5
Can someone please show me why the following is possible?
If
then
I don't understand this last step. In general are you allowed to multiply two integrals together to get a double integral?
Also, another question. The next step it says this is equivalent to
Is this a mistake? I thought you would integrate
from 0 to infinity, and integrate from 0 to ?#167 Re: Jokes » The Panda » 2009-08-29 02:00:24
I like that!
Reminds me of "Residents refuse to go into the bins", which is missing the apostrophy that would give it the intended meaning...and at the same time change the pronunciation of one of the words.
"Eats, Shoots & Leaves"...I have that book. Here are images from it: the cover page and the panda story.
Ah Eats shoots and Leaves, I had to study that book as part of English Language, and I thoroughly disliked it 
#168 Re: Help Me ! » integrate » 2009-08-25 01:24:04
Nevermind this I made a mistake on the first line
#169 Re: Help Me ! » Solving simultaneous equations » 2009-08-24 18:20:30
that one is easily solved by substitution. x is already given for you.
#170 Re: Help Me ! » Financial Mathematics » 2009-08-22 05:33:17
You could also graph it if you use a graphics calculator and find zeros from that.
#171 Re: Maths Is Fun - Suggestions and Comments » Complex Numbers » 2009-08-17 19:33:04
Looking great! Complex numbers is one of the most fascinating topics for me.
#172 Re: Exercises » trig » 2009-08-15 20:24:45
Seeing products of trig terms, the product to sum formula might be useful:
Now, to the other equations,
Adding,
Also, subtracting,
Now if x is in the first quadrant then we get
If x is in the second quadrant then we get
#173 Re: Exercises » Trigonometry Proofs » 2009-08-15 20:04:50
Most trig proofs involve converting 'tan' and 'cot' to 'sin' and 'cos', and lots of fractions!
#174 Re: Help Me ! » Probability Question HELP please » 2009-08-15 16:12:14
C(26,13) is the number of ways of picking 13 objects from 26. There are 26 red cards in a deck, so C(26,13) is the number of ways of picking 13 cards from the 26 red cards.
Perhaps a better way to write it would be
At the top we are showing that we choose 13 cards from the 26 red cards, and 0 cards from the 26 black cards, i.e. all the cards we pick are red. The probability remains the same, since
.C(52,13) is the number of ways of picking 13 cards from the entire deck.
#175 Re: Maths Is Fun - Suggestions and Comments » Define "Simplification" » 2009-08-14 18:36:52
hmm it's kinda hard to define...
Every form is useful for some purpose... and I guess that would be my definition: The simplest form is the form which is most useful for a specific purpose, so the notion is relative.
For example, you could argue that
is more simplified than , but it would be easier to integrate the second rather than the first.And take for example
If you wanted the asymptotes you would go for
but if you wanted to integrate you would go forEven with
, while most people would say that is the most simplified form, that's only because it's easier to evaluate. If we aren't worried about evaluating, but simply math theory, then there's there's no reason why we couldn't say is more simplified.