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#251 Re: Help Me ! » differantiating products and quotients of trig functions » 2005-11-30 19:14:07

The derivative (as you know) is the slope of the function. If two functions, rearranged algebraically, are truly equivalent, then they should also have the same graph, meaning the same slope, or derivative.

At least, that's what I think, but math can be a strange beast and surprise unexpectedly. Can anyone think of an exception?

#252 Re: Help Me ! » Tough College Perms/Combs Question » 2005-11-30 18:41:58

From my College Algebra text:

Permutations: Distinct Objects with Repetition
The number of ordered arrangements of r objects chosen from n objects, in which the n objects are distinct and repetition is allowed, is n^r.

So, in your case, 5^12 = 244,140,625. That's a lot of marbles!

#253 Re: Dark Discussions at Cafe Infinity » Random Words » 2005-11-29 18:10:30

Algodón! Encabezamientos! Pañuelo! Guapote! Fuertote! Descremada! Hedor! Actualización! Nudo!

#254 Re: Help Me ! » Hard Question! Need help! » 2005-11-25 05:02:52

Since ab + bc + ac = 3, let ab, bc, and ac all equal 1. Now, 1³ + 1³ + 1³ + 6(1) = 9.

Is this the lower bound? My intuition says yes, because to decrease one of them, you'd have to increase at least one other, possible more than you decreased the one. However, I don't know how to prove it. Maybe someone else could step in?

#255 Re: Maths Is Fun - Suggestions and Comments » Scientific Calculator » 2005-11-23 18:12:37

6^1953125 ends in: 02220063994332456747346268935437376264942880349499173996030029353486977990646384748721787109376

#256 Re: Dark Discussions at Cafe Infinity » Happy Thanksgiving » 2005-11-23 17:56:57

Thanks! Right back at ya! (Doesn't England celebrate Thanksgiving, too?)

#257 Re: Maths Is Fun - Suggestions and Comments » Scientific Calculator » 2005-11-22 19:16:45

Well, I've hit the limit. I typed 2^195312500 into bc, and it merrily spit "Runtime error (func=(main), adr=15): exponent too large in raise" back at me. There must be an open-source calculator out there that could do it...I may just need to have a look.

#258 This is Cool » Solar Zinc » 2005-11-22 18:56:21

ryos
Replies: 12

I just wrote a research paper about this. I think it's really amazing and cool.

A way to store 60% of the sun's energy in zinc metal, which can be used in zinc-air fuel cells. It (zinc) energy-dense enough to give 150 miles/gallon. I so want one of those.

Linky.

#259 This is Cool » Donald Duck in Mathemagic Land » 2005-11-22 18:52:22

ryos
Replies: 3

I was reading Ganesh's post, "Any Guesses?", and the mention of the Golden Ratio reminded me of this film. It's quite old, from the eighties (or maybe even seventies...I don't know). In it, Donald Duck has an extended adventure inside his head, with an omniscient narrator voice teaching him stuff about math while the stuff he's learning beats him up. It's an original, brilliant, hilarious film, and I loved it as a kid. I wish I had a copy...

That, and the Twelve Tasks of Asterisk.

Does anyone know what I'm talking about?

#260 Re: This is Cool » Poll: thousand million = billion? » 2005-11-22 15:49:20

The UK system is easier because to find out what power of 10 it is, you just multiply the ...illion prefix by 6.

"I don't know what that means...but you still smell like pea soup..."

(The pea soup thing's not an insult, it's a culture reference. And I don't know what it means. smile)

#261 Re: This is Cool » Poll: thousand million = billion? » 2005-11-22 06:43:57

Add six...pschaw. Add three!

10^6 = million
10^9 = billion
10^12 = trillion

Why, you say? Because it fits with the SI system's tendency to add 10^3 to its incremental units. And it's easier to say that a "Gigawatt" is a billion watts than it is to call it a thousand million (which doesn't quite make sense).

Or it could be because I'm American, and refuse to say "colour".

#262 Re: Help Me ! » proof of the sum of squares » 2005-11-21 10:43:28

I didn't learn that one till college algebra...

#263 Re: Maths Is Fun - Suggestions and Comments » Scientific Calculator » 2005-11-21 03:47:21

How did you know I'd be up all night writing a research paper? >:[

Maybe tonight...

#264 Re: Maths Is Fun - Suggestions and Comments » Scientific Calculator » 2005-11-20 04:26:04

2^39062500 ends in: 88796961139264145200433071957141681787109376

I let it run all night, so I don't know how long it took. I'll run 2^195312500 tonight...

#265 Re: Maths Is Fun - Suggestions and Comments » Minesweeper MathsIsFun Style » 2005-11-19 20:15:49

Interesting. My first impression was, "What?" Then, "Oh. Cool!" Then, "There's a lot more times that you must just guess." Then, "I wonder if there are some mad techniques for guessing?"

For example, in regular minesweeper, I would take guesses based on a rough estimate of the probability of a mine (or blank) occupying a certain space. I based it on the number of bombs I knew were in a given area, the total number of bombs left, and total number of squares left. It worked quite well.

I haven't found any similar technique here. smile However, I do feel a lot less like a computer could do a better job at it than I could.

Bugs I've seen so far:
I found that I could drag a flag onto an already-flagged square to run down the number of available flags. I didn't test what happens when it hits zero; I'm guessing you get stuck in sweeper limbo.

Also, it's possible to click and explode mines that have been exposed by a shovel.

It also ran very slow, but that could just be because I'm running 2^39062500 in the background. wink

Mechanical suggestions:
Why limit the number of flags at all?
A "bombs remaining" counter would be nice.
Dragging flags is incredibly tedious. Couldn't you do a platform-neutral modifier-click, like shift, to flag mines?

Visual suggestions:
The horizontal lines are easy to follow, but the diagonals got a little hard on the eyes. Once you get a mishmash of flags out there, there's quite a lot of visual ambiguity in the diagonals--what I mean to say is, the eye has a hard time following the line, and you have to check closely and convince yourself that you're still on the same diagonal you started with. Alternating shading would be awesome.

It would be very useful to grey out or otherwise identify lines that have been "satisfied" so you can block them out of visual scans.

That's it for now.

#266 Re: Help Me ! » a history of mathematics » 2005-11-19 16:10:34

Calculus seems to be the study of "everything we couldn't figure out until Newton and Leibnitz."

The word calculus once meant "stone." I guess whoever named it thought it was heavy, too!

#267 Re: Help Me ! » Straight Line Graph Equations » 2005-11-19 04:44:10

Your first equation is correct. It's in the form y=mx + b, where m is the slope and b is the y-intercept. b was easy to find because it was given to you, but not all problems are so nice. In this case, point-slope form (y = y1 + m(x - x1)) is your friend. So:

y= 3 + 2(x - 2)
y= 3 - (x - 4)
y= 1 + 4.5(x - 2)

etc.

#268 Re: Maths Is Fun - Suggestions and Comments » Scientific Calculator » 2005-11-18 10:26:06

Hey, anything to help the search for a higher power. tongue

#269 Re: Maths Is Fun - Suggestions and Comments » Scientific Calculator » 2005-11-17 14:09:08

2^7812500 ends in: 719540313499900586420401787109376

Oh yeah, logarithms. Duh. Nice job! smile

#270 Re: Help Me ! » Trig Waves question ??? » 2005-11-17 13:04:51

Plain old cosx has a period of 2π. The variable (t) inside of cos is multiplied by 5, which compresses the period. So, the answer is 2π/5

The shift is the quantity subtracted from the variable inside cos. It seems that the shift is divided by the coefficient of t, giving 3π/10. I don't know why this is, but I just varified it with a graphing program and it's true.

#271 Re: Help Me ! » A math problem for kids.....I don't even get it. » 2005-11-17 12:50:32

Assuming that all servings contain an equal measure of all parts, you can average the calories accross the pieces.

Volume of a cylinder: V = πr²H
r = 1/2d = 9 3/8 = 75/8 in.
H = 2(3.5) + 5/8 = 7 5/8 in. = 61/8 in.

Total volume of the cake: π(75/8)²(61/8) = 2105.388631 in³

Volume of chocolate frosting: V = π(9)²(1/4) = 63.61725124 in³

Volume of buttercream frosting: We'll need three cylinders for this one.
V of top = π(9)²(3/8) = 95.42587685 in³
V of sides = [ π(75/8)²(61/8) ] - [π(9)²(61/8) ] = 165.0624687 in³
total V of b. frosting: 260.4883456 in³

Volume of cake portion = 2[ π(9)²(3.5) ] = 1781.283035 in³

Calories from cake: 1781.283035 * 140 = 249379.6248 cal
Calories from chocolate frosting: 63.61725124 * 320 = 20357.5204 cal
Calories from buttercream: 260.4883456 * 470 = 122429.5224
Total calories in cake: 392166.6676 cal

Number of 800-calorie servings = 392166.6676/800 = 490, with a little less than a fourth left over.

All that talk of cake is making me hungry...

#272 Re: Help Me ! » Diffrentiation.........HELP ME PLEASE!!!! » 2005-11-17 08:01:04

Don't thank me too soon mike, I just realized I made a mistake! yikes

In the second problem, I factored e^3x out of e^6x and put the answer as e^3x. But it's e^2x!

Sorry! I should have caught that...

#273 Re: Maths Is Fun - Suggestions and Comments » Scientific Calculator » 2005-11-17 05:39:49

2^12500 ends in: 38413079360370299109376
2^62500 ends in: 35077828320112175538998220106163281524347109376
2^312500 ends in: 2616427893377447294587109376
2^1562500 ends in: 28458279155722394465726629493874312276145787109376

That last answer was about 125 pages long! (Well, terminal screens anyway)
Here's a puzzler for you. If the program wraps the answer to 69 numbers per line, how many lines are in the answer to 2^1562500?

I wouldn't know how to solve that!

#274 Re: Help Me ! » Diffrentiation.........HELP ME PLEASE!!!! » 2005-11-16 13:15:10

y=xSin( √ 2x-1) + log2 (3x+1)

We need the product and chain rules for the first piece.
(x)′ = 1
( sin (x) )′ = cos (x)
( √[x] )′ = 1 / 2√[x]
( 2x-1 )′ = 2

So, sin( √ 2x-1)′ = cos(√[2x - 1]) * (1 / 2√[2x - 1]) * 2
= cos(√[2x - 1]) / √[2x-1]

Making the derivative of the first chunk = x{ cos(√[2x - 1]) / √[2x-1] } + sin( √ 2x-1)

Now for the second chunk, we just need the chain rule, but first we need to rewrite the logarithm:

log2 (3x+1) = ln(3x+1)/ln(2) = [1/ln(2)] * ln(3x+1)
( ln(3x+1) )′ = 3 / (3x+1)

The whole nasty first derivative is x{ 2cos(√[2x - 1]) / √[2x-1] } + sin( √ 2x-1) + 3 / [ ln(2)(3x+1) ]

The second derivative requires application of the chain, product, and quotient rules and should be quite awful. I'm sorry I don't have time right now to work it out for you.

y={1+cos(1-x)/e^3x} + ln(2x+5)

Same approach as before. First part:
( cos(1-x) )′ = sin(1-x)
(e^3x)′ = 3e^3x

1 + { (e^3x)(sin(1-x)) - (cos(1-x))(3e^3x) } / e^6x
factor out an e^3x:
1 + (e^3x/e^3x) { sin(1-x) - 3cos(1-x) } / e^3x
= 1 + { sin(1-x) - 3cos(1-x) } / e^3x

Second part:
(ln(2x+5))′ = 2 / (2x + 5)

Put them together:
1 + { sin(1-x) - 3cos(1-x) } / e^3x + 2 / (2x + 5)

Again, sorry I don't have time for ƒ″. I hope I helped point you in the right direction.

#275 Re: Maths Is Fun - Suggestions and Comments » Scientific Calculator » 2005-11-15 05:25:48

2^2520

39408424552214162695348543183638915172819172249751642655322154182349\
33676588009610655644786388200003560563883371670355420740089454019139\
50236214360506399705231203021164366069389563701733455174652493802096\
52827965938125948350891617678251689261632215488187059650565457777432\
98081872565023704682568753763162781359379857881608885188091378378731\
80086327183792757748702946460720720770436177477377229784500022657580\
65723362838393013791461968400922079126708976855218290361860314695008\
42192427800725780716480012657266798737517723023431143584285521349919\
38056446803917216962620267368806273089867659639177213488960155211698\
14921103068177978857814105435927428955641140043659870492782127521488\
14889702185765573255518895775073409289563384104009610960263526424138\
31783448576

Hey ganesh, what kind of computer are you using? Unix and lookalikes have a program called bc; it's an arbitrary precision calculator. Just for fun, I typed 2^312500 into it, and it spat out the answer, full and complete, in about 5 seconds. Not bad.

If you happen to be running Windows, there is a program called Cygwin (I think) that emulates a unixy environment. You might give it a look.

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