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  Discussion about math, puzzles, games and fun.   Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫ • π ƒ -¹ ² ³ °

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#101 Re: This is Cool » Quantum computer gives answer when not running? » 2006-02-23 11:40:12

"A non-running computer produces fewer errors,"

Is that a joke? Computers don't produce errors, the people who programmed (and designed) them do.

In fact, the whole concept of a "non-running program" seems a bit silly. The computer could not give an answer in the absence of the program. Even if the photon doesn't actually run through the program, it's still the program's quantum nature influencing the photon's quantum nature (AFAICT). So, though the program runs not, yet it still runs.

#102 Re: Puzzles and Games » fun with logic » 2006-02-23 11:28:13

This is kind of lame, but bear with me.

Lay them on their sides so that they form a square. Two glasses on opposite ends of the square are one glass-length apart.

"Wait," you say, "what about the glasses that are touching? They're much closer to each other than those that aren't!"

Aha, but with logic, I can distort reality. You see, a part of those glasses (the opposite end from the one that's touching) is one glass-length away, so it can be said that all of the glasses are equidistant.

I know, I know...lame. But it's all I've got.

#103 Re: Help Me ! » differential equation xy"+y'-mxy=0 » 2006-02-23 06:51:18

What do you need to do to it?

I'm a little rusty on diffEQ (what I've had of them, anyway), but don't you need to integrate both sides? In this case, you'd have to do it twice.

I've also not had multivariable calculus, so I'd need to separate the variables.
So, solving for x, we find that
x = -y′ / (y″ - my)

Integrating both sides gives:
1/2 x² + C = ∫[ -y′ / (y″ - my) ]

The integral on the right evades me. Sorry. sad

#104 Re: Maths Is Fun - Suggestions and Comments » Algebra - Introduction » 2006-02-22 18:50:50

I also think they're well done.

While you're treating order of operations, a bit on the distributive property would also be beneficial to the multiplication page, since it's not readily apparent at first that if you multiply both sides by something, you have to do it to everything on both sides.

I remember having a tough time with the distributive property in 7th grade. Surprisingly, a simple, clear explanation of this concept was not available to me. It seemed like this mysterious thing with a fancy name instead of the simple, intuitive concept that it is.

#105 Re: Puzzles and Games » Random Quiz » 2006-02-20 09:31:31

9. An hp is 745.7 W. (I had to look that up; who memorizes unit conversions?)

#106 Re: Puzzles and Games » Random Quiz » 2006-02-20 09:29:15

4) Barbados
8) SCUBA is Self Contained Underwater Breathing Apparatus. ELISA could be (just guessing here) English Language Internationally Standardized Aptitude test.
11) A nautical mile is like 6080 ft as opposed to a mile's 5280. I don't know without looking it up, but I believe it derives from the old (still-used) measure of speed, the "knot". Knots used to be measured by throwing a float attached to a knotted rope out the back of a ship, then counting how fast the knots reeled out.
12a) Dichloro-diphenyl-trichloroethane is a banned pesticide. (Go organic chemistry!)

#109 Re: Help Me ! » need help finding the derivative » 2006-02-20 09:05:19

Or, using the power rule:

(1/x) = x^-1
(x^-1)′ = -1(x^-2) = -1/x^2

#110 Re: Puzzles and Games » Acronym Puzzle » 2006-02-19 18:02:21

Darth and Justlooking are correct. smile

I'm still clueless about g) 1 FBOHZIAG.

#111 Re: Help Me ! » Surfaces of Revolution » 2006-02-19 07:28:27

And if you have a shape of known cross section, the integral becomes much simpler.

Like this paraboloid, for example. The equation is z = -(x² + y²) + 10. The cross section is a circle of radius √[-z + 10] = x² + y². The circumference is then 2π√[-z + 10] = x² + y².

You then integrate the circumference multiplied by the differential of z, dz.

In coming up with this example, I've realized a deficiency in my instruction that I'm sure will be filled at a later date, but I want to know now. big_smile How would I set up said integral? Always before when I've done problems like this, they've given me a nice neat expression for r. Now...I don't know.

#112 Re: Puzzles and Games » Acronym Puzzle » 2006-02-18 21:10:51

These are fun, 23 JLFTM!

n) 8 BIAB

o) 7 DYTHY

p) 1.21 GWTTBTTF

q) 3 ICFIAC

r) 6 SOAD

s) 52 PCPD

#113 Re: Help Me ! » integration trouble » 2006-02-18 18:27:58

irspow wrote:

You are correct ryos, it wouldn't really matter if this problem was to be solved by a computer.  The two answers are indeed equal.  I was thinking however about the scenario in which the result was going to be an integral (pun not intended) part of one's work.

And that, in a nutshell, is the difference between scientists and engineers. tongue

#114 Re: Puzzles and Games » Acronym Puzzle » 2006-02-18 18:17:52

Looks like I was wrong on d. It fit the acronym, but I didn't bother to check the number.

Wikipedia wrote:

The Winchester Round Table, which dates from the 1270s, lists 25 names of knights. Every year, Arthur had the Knights return to Camelot on Pentecost. Different stories had different numbers of knights, ranging from only 12 to 150 or more.

#118 Re: Help Me ! » integration trouble » 2006-02-17 13:28:22

As my Calc teacher once said, "Machines sometimes give us some interesting answers..."

Remember that computers have no intuition. I'm sure they've got their algorithms such that if a function can be integrated, they will integrate it somehow. It's doubtless not the best at selecting the proper method for every integral, and for some it probably doesn't have a "best method." But if you have the attention span of a computer, it works just fine.

Or maybe somebody taught the computer about artificial job security...

Anyway, I guess this is a case where a calculator isn't the best tool after all. Though, if you're using computers end to end, then the computer doesn't care how ugly the function you ask it to evaluate is...

#120 Re: Help Me ! » Definite integrals by parts » 2006-02-17 05:21:52

\mbox{The first post, mathsy. 8O)}

#121 Re: Help Me ! » Definite integrals by parts » 2006-02-16 16:44:54

Stupid typos. Again. It's fixed now.

As to where I got it, umm...read my posts again.

#122 Re: Help Me ! » Definite integrals by parts » 2006-02-16 15:06:05

I'm advanced eh? I guess I'm good at sounding smart or something. I'm pretty sure I'm in the same place (Calculus 2) as you, mikau.

Anyway mikau, my question (which both you and irspow answered the same way) is this, using your example:

xe^x - e^x

Is it:
(3e³ - 2e²) - (e³ - e²)

...or is it:
(3e³ - e³) - (2e² - e²)

...? Actually, working through the two, they both come out the same. Is this always the case, though? Let's see:

Let h(x) = uv and g(x) = ∫u′v, then assert:
[h(b) - h(a)] - [g(b) - g(a)] = [h(b) - g(b)] - [h(a) - g(a)]

h(b) - h(a) - g(b) + g(a) = h(b) - g(b) - h(a) + g(a)

So it is. Ok, dumb question. *smacks self*

Edit: on the plus side, this post made me finally learn LaTeX, or at least enough to write it.

#123 Re: Help Me ! » Definite integrals by parts » 2006-02-16 12:40:50

Yeah, it's straight from my book. And maybe it's because I learned from that book, but I find it clearer than Ricky's version.

There was an error in it though, which I've now corrected.

#124 Help Me ! » Definite integrals by parts » 2006-02-16 08:55:04

ryos
Replies: 11

My book says that the definite version of the integration by parts formula is:

When actually evaluating the integral, I'm not sure how to interpret this. I think it could be either:

...or, it could be:

Is it one of those, or something else entirely?

(Good gads, LaTeX competes with regular expressions for ugliest mishmash of punctuation ever.)

#125 Re: Maths Is Fun - Suggestions and Comments » Pascal's Triangle » 2006-02-15 19:21:09

But you didn't explain what it's good for...polynomial expansion!

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