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#3601 Re: Help Me ! » Help... » 2006-01-17 03:14:03

A lot of people who are serious about learning seem to prefer college to highschool. In college, kids are there because they chose to go there. In high school, they're there because they're required by law. Of course theres always the idiot who goes to college to party it up and all but they usually don't last long since there grades are bad.  At least thats what I heard.

The vast majority succeed in doing both.  Myself included.

#3602 Re: Help Me ! » Concentration of brine in cooling » 2006-01-16 04:11:27

You use a different method for solving differential equations than I'm used to, and quite an awkward one at that.  See if you can follow this, and if you want, I can try to find the error in your steps.

Integrate both sides

#3603 Re: Help Me ! » A really obvious but interesting question » 2006-01-16 03:43:14

rickyoswaldiow, (s)he's asking:

Why do u make 1/2 into 2/1?

Not just how.  I guess one reason why is because it's hard to divide fractions, but it's easy to multiply them.  I don't think that one was ever said.

#3604 Re: Help Me ! » Very interesting problems.. » 2006-01-15 05:53:59

There must exist i0 element of N : for every i>=i0 a_(i+2)-a_i<i.

if n=2 we get

F(i+2)-F(i)>i, i>2;

You haven't defined N, a_, or F() or n.  I have no idea what these are.

#3605 Re: Help Me ! » Looks simple but actually quite hard » 2006-01-15 05:50:36

Boy, if only I could.  But Texas Instruments protects stuff like that.

But it isn't krisper.  Factoring a quartic equation is only an exercise if you are bored or insane.  No math teacher woudl ever have you do this, nor would you learn anything from doing it.

#3606 Re: Help Me ! » Looks simple but actually quite hard » 2006-01-15 04:15:04

I'm just that good.

Nah, I used a ti-89.  It uses some fairly complex algorithms to do it, most of which would be unusable by a human.

#3607 Re: Help Me ! » how to differente this? » 2006-01-15 03:45:36

The chain rule strikes again!

Using the chain rule, the derivative of 1 - p is -1.

#3608 Re: Help Me ! » please help me :( » 2006-01-15 02:41:40

You can do all of the above without a calculator.  It's just multiplying decimals.  Ugly, but it works.

#3609 Re: Help Me ! » Looks simple but actually quite hard » 2006-01-15 02:39:01

Square both sides, mutiply by the denominator, then subtract.  You end up with:

x^4 - x^3 - 10x^2 - 5x + 7 = 0

Which factors into:

(x^2 + x - 1)(x^2 - 2x - 7) = 0

And you use the quadratic from there.  It comes out to (-√5 + 1)/2, (√5- 1)/2, -2√2 + 1, and 2√2 - 1.

#3610 Re: Help Me ! » Please Help Me With A Permutation Question!!! » 2006-01-15 02:00:51

That's my biggest pet peeve ever.  Reading a question (especially math!) wrong.  I absolutely hate when I do it.

#3611 Re: Help Me ! » Very interesting problems.. » 2006-01-15 01:53:46

Boy, I thought this thread was finally dead...

But I was wondering if there exist different proof without guessing.

I think there is, if you know enough about Hamiltonian Circuits.  I don't.  But I wouldn't quite call it guessing, just making observations.

#3612 Re: Help Me ! » please help me :( » 2006-01-14 12:47:37

[Hehe, I beat you by 2 seconds, irspow!]

Hey, you don't have an "Edited by...." under your post.

[Nor do you. Oh, the joys of being a mod. wink ~mathsy]

#3613 Re: Help Me ! » how to differente this? » 2006-01-14 12:45:51

Whoops, I wrote the solution, not the problem.  What I meant was:

(1-p)-¹

#3614 Re: Help Me ! » how to differente this? » 2006-01-14 11:51:57

You do have f(x)/g(x), but you also have k/g(x).  Like I said, either way works.  You could also multiply them:

f(x) * g-¹(x), and use the product rule.

#3615 Re: This is Cool » Implicit Graphs » 2006-01-14 10:18:02

Red:

cos(xy) = 0
cos(θ) = 0 when θ = pi/2 or 3pi/2.  So xy = pi/2, y = pi/(2x) or xy = 3pi/2, y = 3pi/(2x).

Green:

sin(xy) = 0
sin(θ) = 0 when θ = 0 or pi.  So xy = 0, x = 0 or y = 0, or xy = pi so y = pi/x.


Blue:

tan(xy) = 0
tan has periodic 0's at n*Pi.  This means that xy = n*Pi, so y = nPi / x, where n is any integer. 
I believe your graph of this is wrong, it should just be curves of y = pi/x, 2pi/x, 3pi/x, etc.

And remember that each one of these is periodic, which means it repeats.

What software did you use to make them?

#3616 Re: Help Me ! » vectors » 2006-01-14 09:51:00

First, we need to make these vectors.

The vectors x direction is cos(θ).  The y direction is sin(θ).

<x, y> = <cos(90°), sin(90°)>

However, this gives us a magntidue of 1.  We don't want 1, we want 3:

3 * <cos(90°), sin(90°)> = <3*cos(90°), 3*sin(90°)>

And that gives us it in vector form.

Now do the same for the other vector, and you'll have to vectors:

v1 = <x1, y1> and v2 = <x2, y2>

v1 + v2 = <x1+x2, y1+y2>
v1 - v2 = <x1-x2, y1-y2>

#3617 Re: Help Me ! » Please Help Me With A Permutation Question!!! » 2006-01-14 09:44:19

I don't think that's right.

Let's start with one digit.  You can go from 0 to 9, so you have 10 choices.

Now two digits.  Each are 0-9, with which you can represent all numbers from 00 to 99, 100 choices.

Same for three digits.  1000 choices.

And so on.  The answer is 10^x, where x is the number of digits.  10^6 in this case.

#3618 Re: Help Me ! » how to differente this? » 2006-01-14 06:07:11

Ah, sorry, should have probably been clearer.

For the first question, it's much easier to do:

1/(1-p)^2 = (1-p)^-2

But either way works.  And besides, it's much better know multiple ways to differentiate.

#3619 Re: Help Me ! » Calculating Angular Acceleration. Im struggling a bit. » 2006-01-14 05:25:26

Hopefully I will also be posting some answers soon.

Huh?  If you think you can do the problems, try them first before asking for help.  If you want us to check your answers, then post all your work.  It makes it easier for us to help you.

#3620 Re: Help Me ! » how to differente this? » 2006-01-14 05:22:25

Low d' Hi minus Hi d' Low, all over the denominator squared we go.

#3621 Re: Help Me ! » Integrating fractions » 2006-01-13 01:39:15

Bad algebra.  It's not (4x)^5, it's 4 (x)^5.

A good way to rewrite it is:

4^1 * x^5

So 3 / (4^1 * x^5) = 3*4^-1 * x^-5.  Of course, this is just (3/4) x^-5.

Edit: Oh, and my point in my first post was that you should just take the fraction out before you integrate, then multiply.  I forgot to say that though.

#3622 Re: Help Me ! » Integrating fractions » 2006-01-12 23:44:17

∫2x^2 - x/2

∫2x^2 - ∫x/2

2∫x^2 - 1/2 ∫ x

2/3 x^3 - 1/4x^2 + C

#3623 Re: Puzzles and Games » Real life architectual problem » 2006-01-12 19:06:32

Ok, here's me thinking outside the box.

Take a protractor, measure the angle of the triangle, use this and tan to find A.

Problem solved.

#3624 Re: Puzzles and Games » Real life architectual problem » 2006-01-12 18:49:32

swear

Then I give up.  I'm not quite sure if it is possible since you don't know any of the angles.

Of course, if your father measured one single angle, he would have saved me 30 minutes of work. hmm

#3625 Re: Help Me ! » trouble with trig » 2006-01-12 18:27:25

May I also suggest an image which may help you:

big_smile

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