## Search found 61 matches

- Tue Feb 19, 2013 3:58 am UTC
- Forum: Individual XKCD Comic Threads
- Topic: 1175:"Moving Sidewalks"
- Replies:
**78** - Views:
**22488**

### Re: 1175:"Moving Sidewalks"

The arrows were correct the way it was for the United States. People walk on the right side in hallways and sidewalks. Actually, in the United States, people are supposed to walk on the left side of the street to face oncoming traffic because it is easier to see what's coming from the front than th...

- Mon Feb 18, 2013 9:02 pm UTC
- Forum: Individual XKCD Comic Threads
- Topic: 1175:"Moving Sidewalks"
- Replies:
**78** - Views:
**22488**

### Re: 1175:"Moving Sidewalks"

They do have various solutions to making high-speed and variable-speed walkways. http://en.wikipedia.org/wiki/Moving_sidewalk#High-speed_walkways The most practical is probably this one from Toronto's Pearson airport . "The walkway moves at roughly 2 km/h when riders step onto it, speeds up to ...

- Mon Feb 18, 2013 3:02 pm UTC
- Forum: Individual XKCD Comic Threads
- Topic: 1175:"Moving Sidewalks"
- Replies:
**78** - Views:
**22488**

### Re: 1175:"Moving Sidewalks"

The arrows were correct the way it was for the United States. People walk on the right side in hallways and sidewalks. As it appears that Mr. Munroe's intention was for the two to be going to on the correct direction, he should have moved them instead of the arrows. Edited to Add: If this was a Mini...

- Mon Feb 18, 2013 1:57 pm UTC
- Forum: Individual XKCD Comic Threads
- Topic: 1175:"Moving Sidewalks"
- Replies:
**78** - Views:
**22488**

### Re: 1175:"Moving Sidewalks"

My preference for innovation in ambulation is Improv Everywhere 's concept of separate Tourist Lanes and New Yorker lanes so that lollygaggers can get out of my way: http://improveverywhere.com/2010/06/08/the-tourist-lane/ As for the comic, In the United States, we walk on the right side of sidewalk...

- Wed Aug 31, 2011 3:13 pm UTC
- Forum: Mathematics
- Topic: Probability Problems
- Replies:
**10** - Views:
**1758**

### Re: Probability Problems

I'm not sure why you're using combinations and why you're dividing by the total number of combinations of people. Restating what Yakk said, if you know how may total combinations are possible and you know how many combinations meet your criteria, what fraction of those total combinations meet you c...

- Tue Aug 30, 2011 5:50 pm UTC
- Forum: Mathematics
- Topic: Probability Problems
- Replies:
**10** - Views:
**1758**

### Re: Probability Problems

For #2, show your work on how you come up with your answer without combinations, showing the three fractions you'd multiply together. If you take the reciprocal of that, can you convert those terms into a single combination? Problem #2 is simplest because you're selecting all the boys, \binom{3}{3} ...

- Fri Aug 26, 2011 5:04 am UTC
- Forum: Mathematics
- Topic: It's not a square root function, but... what is it?
- Replies:
**13** - Views:
**4005**

### Re: It's not a square root function, but... what is it?

To correct your original equation, you mean you're doing: x + (y - x * x) / (2 * x + 1) Where x^2 is the nearest perfect square to y. Because then if we say y = 124, x = 11 you'd get the answer you're producing. You can even iterate this to get towards the exact root. Define the sequence of numbers...

- Sun Nov 07, 2010 5:24 pm UTC
- Forum: Mathematics
- Topic: What are these numbers called?
- Replies:
**6** - Views:
**1299**

### Re: What are these numbers called?

Of course, "relatively minuscule" isn't a well defined term How about "p is the i th short prime if the decimal expansion of 1/p has a period of i and p is the largest number that has such a period." Plug the first few into the OEIS and see what happens. They have A005422: Large...

- Sun Nov 07, 2010 4:11 pm UTC
- Forum: Mathematics
- Topic: What are these numbers called?
- Replies:
**6** - Views:
**1299**

### Re: What are these numbers called?

So does that make 13 a "part reptend prime"? Does it have a name other than non-full reptend prime? Is there a special name for prime numbers that have a relatively minuscule period on their repeating decimal? For example, 37 (period 3), 101 (period 4), 41 & 271 (period 5), 73 & 13...

- Wed Oct 06, 2010 2:30 pm UTC
- Forum: Mathematics
- Topic: Putnam Studying
- Replies:
**5** - Views:
**1028**

### Re: Putnam Studying

As with all math competitions, it'd be good to know as many characteristics and "neat" features about the number 2010 as possible. I spent a few minutes looking for what those features might be without much luck. Unlike some tests, it's not important to have the prime factorization of the ...

- Wed Oct 06, 2010 2:40 am UTC
- Forum: Mathematics
- Topic: Putnam Studying
- Replies:
**5** - Views:
**1028**

### Re: Putnam Studying

The obvious answer is to look at previous exams: http://amc.maa.org/a-activities/a7-problems/putnamindex.shtml I never took the Putnam, but assuming it's like the USAMO, you won't get repeat questions, but you'll train your mind to think like the test writers and learn what topics the test might cov...

- Thu Sep 23, 2010 4:20 am UTC
- Forum: Mathematics
- Topic: Trick math problems?
- Replies:
**24** - Views:
**4592**

### Re: Trick math problems?

It doesn't grow at a constant rate, and it is not at the rate you mention at the time stated. You may have missed that the second car stands still for the first hour. You're entirely correct as i missed the second car standing still. If they started at the same time the rate would be the constant 1...

- Thu Sep 23, 2010 2:39 am UTC
- Forum: Mathematics
- Topic: Trick math problems?
- Replies:
**24** - Views:
**4592**

### Re: Trick math problems?

I still remember the first time that I felt that I had "invented" something of mathematical significance, that made the following problem (or one similar to it) much simpler than my AP Calculus teacher had intended. Two cars are at a point. One car heads east at 20 mph. An hour later, the...

- Thu Sep 23, 2010 2:27 am UTC
- Forum: Mathematics
- Topic: Probability help (DNA bases)
- Replies:
**9** - Views:
**1038**

### Re: Probability help

Copy the exact wording of your non-mark assignment (and, ideally, the course description/level) out into the forum. Because your problem is a technicality if anything. In short, your "summary" is wrong. Hmm, I think I figured out what the technicality is, and it doesn't require that there...

- Sun Sep 19, 2010 4:18 pm UTC
- Forum: Mathematics
- Topic: Rational number are periodic : proof
- Replies:
**12** - Views:
**2292**

### Re: Rational number are periodic : proof

Hi, i'm stuck with how to prove that a fraction of integers a,b : a/b will have a periodic decimal development. Has someone a simple proof for this ? That depends on whether a decimal that terminates counts as having a "periodic decimal development." I agree with taking the long division ...

- Fri Sep 17, 2010 2:58 am UTC
- Forum: Mathematics
- Topic: Runner Question
- Replies:
**8** - Views:
**1513**

### Re: Runner Question

Ok, this question was more complicated than I thought, but I think I do get it, thanks guys! Admittedly, I actually don't really get why 2/5 doesnt equal 4/10? Could someone explain this clearly? xD I'm not too good at math. I thought my previous description was pretty good. Let me try a different ...

- Tue Sep 14, 2010 3:39 am UTC
- Forum: Mathematics
- Topic: Runner Question
- Replies:
**8** - Views:
**1513**

### Re: Runner Question

Hold on - suppose we took the opposite approach. Placing second in a race of five could mean that, you beat 3 other runners. So, by doubling the size of the group, you would beat 6 other runners and place 4th, not 3rd. What gives? You're more than doubling the number of opposing runners (4 to 9). U...

- Mon Sep 13, 2010 2:44 pm UTC
- Forum: Mathematics
- Topic: Appoint all the pairs
- Replies:
**12** - Views:
**1543**

### Re: Appoint all the pairs

There’s a fairly recent thread in the Logic Puzzles forum about numbers similar to this. The last puzzle I posed there has gone unanswered for a week. My follow-ups were to be, what conditions on positive integers A and B are necessary and sufficient to guarantee that a number of the form \sqrt{A±\...

- Sun Sep 12, 2010 6:54 pm UTC
- Forum: Mathematics
- Topic: Appoint all the pairs
- Replies:
**12** - Views:
**1543**

### Re: Appoint all the pairs

Foamy wrote:[math](k+\ell)+(\sqrt{4k\ell})=4+\sqrt{7}[/math]

Look at that more closely (parens added for emphasis).

What does that tell you about the values of [imath]k[/imath] and [imath]\ell[/imath]?

- Sat Sep 11, 2010 3:34 pm UTC
- Forum: Logic Puzzles
- Topic: Poker hands
- Replies:
**26** - Views:
**4277**

### Re: Poker hands

Interestingly, the worst possible situation for you in a Texas Hold'em game (where you are beaten by the greatest number of hands) would be to hold 2-2 against a board of 3-3-4-4-5. The only had you chop against would be the other 2-2. All other hands beat you. Yet i've seen two people call on the ...

- Thu Sep 09, 2010 3:55 am UTC
- Forum: Mathematics
- Topic: Help with a math problem?
- Replies:
**7** - Views:
**1564**

### Re: Help with a math problem?

Very good. I didn't want to give too much away. I realize, in retrospect, that my wording have provoked a wrong answer as i focused the obtuse part of the question more than the equally important triangle part. Good catch. I recall making the same mistake of counting non-triangle forming tuples in t...

- Tue Sep 07, 2010 5:20 am UTC
- Forum: Mathematics
- Topic: Help with a math problem?
- Replies:
**7** - Views:
**1564**

### Re: Help with a math problem?

First, do you know how to tell whether a triangle is obtuse given its sides? I didn't know this offhand, but i figured it out after considering what the Pythagorean Theorem says. Trigonometry would also work, but it's unnecessary and you probably haven't learned that yet. Then, all you have to do is...

- Mon Sep 06, 2010 12:45 pm UTC
- Forum: Individual XKCD Comic Threads
- Topic: 0789: "Showdown"
- Replies:
**46** - Views:
**13323**

### Re: "Showdown" Discussion #789

The tumbleweed then tried to roll off into the sunset, but due to the Old West's placement north of the subtropical ridge, the prevailing winds were in the wrong direction Surely the show down was at high noon, so the tumbleweed has quite a long wait for the sunset! That would be one really dull fi...

- Mon Sep 06, 2010 5:53 am UTC
- Forum: Individual XKCD Comic Threads
- Topic: 0789: "Showdown"
- Replies:
**46** - Views:
**13323**

### Re: "Showdown" Discussion #789

Is the sun circling to the south? Panel 4 is from a different angle, judging by the position of the two men and by the background, but the sun is still straight ahead. I probably wouldn't have noticed, but the alt text drew our attention to the position of the sun. Showdown at high noon. In the Nor...

- Mon Aug 30, 2010 9:36 pm UTC
- Forum: Mathematics
- Topic: Mental Calculation World Cup
- Replies:
**1** - Views:
**829**

### Re: Mental Calculation World Cup

It's worth noting that the person to win the overall combination competition was an 11-year-old Indian girl.

- Mon Aug 30, 2010 8:20 pm UTC
- Forum: Mathematics
- Topic: Mental Calculation World Cup
- Replies:
**1** - Views:
**829**

### Mental Calculation World Cup

Out of curiosity, i looked at the Mental Calculation World Cup to see how it was designed and what the questions were: ⋅ Scoring rules: http://www.recordholders.org/downloads/worldcup/rules-en.pdf ⋅ Results: http://www.recordholders.org/en/events/worldcup/2010/results.html ⋅...

- Mon May 18, 2009 4:47 am UTC
- Forum: Individual XKCD Comic Threads
- Topic: 0585: "Outreach"
- Replies:
**119** - Views:
**36632**

### Re: "Outreach" Discussion

For whatever reason, from the first row of panels, i thought it was going to play on the scuba diver in the tree urban legend, though i wasn't sure if it was going to be a shark or an actual scuba diver (who happened to have the bad timing to be above the device when it popped free). Of course, the ...

- Wed May 13, 2009 4:00 am UTC
- Forum: Mathematics
- Topic: Algorithm and a Question
- Replies:
**3** - Views:
**710**

### Re: Algorithm and a Question

Step 1: It's simple to determine the overall sum of x1 + x2 + x3 + x4, correct? Step 2: Given that, do you know how you can figure out what the sum of just x2 + x3 + x4 is? If so, subtract Step 2 from Step 1 and you have the value of x1. Once you have that, use the values of p1, p2, and p3 to get th...

- Tue May 12, 2009 10:56 pm UTC
- Forum: Mathematics
- Topic: Help! I'm trying to get a 14 year old interested in Maths.
- Replies:
**52** - Views:
**7124**

### Re: Help! I'm trying to get a 14 year old interested in Maths.

Do a substitution of alphabet with the first 26 prime numbers. Then, make a word by taking the corresponding prime numbers and multiplying them together. Of course, this leads to some very large numbers, but if you are careful about word choices, it can be manageable. Possibly the best way to make ...

- Sat May 02, 2009 4:20 pm UTC
- Forum: Mathematics
- Topic: A series with no primes?
- Replies:
**7** - Views:
**1386**

### Re: A series with no primes?

I wonder if looking at proofs involving the periodicity of repeating decimals might help you prove this. The reason i knew your conjecture was true for 2 < n < 11 was from the repeating decimal expansions of various 1/p, where p is a prime. 1001 = 7 * 11 * 13 . Since 999 * 1001 = 999,999, that means...

- Sat May 02, 2009 12:07 am UTC
- Forum: Mathematics
- Topic: number theory solutions
- Replies:
**17** - Views:
**1744**

### Re: number theory solutions

3) In the expansion of 100!, how many zeroes are at the end? How many are at the end of the binary expansion of (100!)? If I've worked it out correctly, the general solution for n! is approximately n/4. I'm thinking that it's the largest integer smaller than n/4, but I'm not sure. You've worked it ...

- Fri May 01, 2009 5:02 pm UTC
- Forum: Mathematics
- Topic: Your favourite calculator
- Replies:
**43** - Views:
**5734**

### Re: Your favourite calculator

I had and still use a HP48SX. RPN calculators work much more like way my brain does arithmetic in that i'll break the problem down and solve intermediate values, push those on to my stack, work on different sections of the problem, before returning to that intermediate solution and incorporating it....

- Wed Apr 29, 2009 5:13 am UTC
- Forum: Individual XKCD Comic Threads
- Topic: 0575: "Tag Combination"
- Replies:
**76** - Views:
**22034**

### Re: "Tag Combination" Discussion

I don't understand the alt text, probably because I'm tired. What tune does that go to and why is it funny? It goes with the tune of "i touch myself" and is funny because it has the words (essentially) from "come on eileen." The lyrics are more from I Touch Myself than Come On, ...

- Wed Apr 29, 2009 4:23 am UTC
- Forum: Individual XKCD Comic Threads
- Topic: 0575: "Tag Combination"
- Replies:
**76** - Views:
**22034**

### Re: "Tag Combination" Discussion

I wish I understood music.... Come On, Eileen is from the fantastic group Dexy's Midnight Runners. They were a one-hit wonder in the US (hitting #1 for one week), but had a few other top 10 songs in the UK. I highly recommend the following, for anyone who likes soul, celtic, rock, ska, rhythm and b...

- Mon Apr 06, 2009 5:35 pm UTC
- Forum: Mathematics
- Topic: help to solve this
- Replies:
**15** - Views:
**1780**

### Re: help to solve this

There is a problem with the recursive solution. Agreed. If i needed this as a permanent function to be used more than once, I wouldn't actually attempt recursion to that depth. This seemed to be the most intuitive way to explain how to find the solutions. Since he was referring to using Excel, it d...

- Mon Apr 06, 2009 1:25 pm UTC
- Forum: Mathematics
- Topic: help to solve this
- Replies:
**15** - Views:
**1780**

### Re: help to solve this

Assuming you needed to find every possible solution (not just a single valid one), my natural instinct would be to do a recursive function in C/C++. First sort the values from largest to smallest. The function would work something like: #define ENTRY_COUNT 175 int sorted_values[ENTRY_COUNT] // The s...

- Mon Apr 06, 2009 3:28 am UTC
- Forum: Mathematics
- Topic: Tricki has been launched.
- Replies:
**9** - Views:
**1342**

### Re: Tricki has been launched.

That's not what the Tricki is about at all. The Tricki is directed towards professional mathematicians who need ideas for proofs. The two-digit multiplication example is demonstrative, not serious advice. Right, but what kind of professional mathematician needs an essay to explain that to solve pro...

- Mon Apr 06, 2009 3:08 am UTC
- Forum: Mathematics
- Topic: Tricki has been launched.
- Replies:
**9** - Views:
**1342**

### Re: Tricki has been launched.

I'm surprised at the initial subjects they have chosen to tackle. From the name, i figured there'd be a wide selection of mental arithmetic tricks. They do mention a few tips about multiplying two-digit numbers , but that's about it. That page then wanders off into non-mental arithmetic subjects. Fo...

- Fri Apr 03, 2009 4:56 am UTC
- Forum: Individual XKCD Comic Threads
- Topic: 0564: "Crossbows"
- Replies:
**171** - Views:
**69003**

### Re: "Crossbows" Discussion

If only it were so awesome like that, though I prefer the longbow myself. For lab/office use? At close quarters, you should prefer the relative maneuverability, shorter draw, and ability to fire around corners that a crossbow provides. For example, if you need to overturn a desk and take cover behi...

- Thu Apr 02, 2009 4:55 am UTC
- Forum: Mathematics
- Topic: How many digits of Pi do you know?
- Replies:
**445** - Views:
**108585**

### Re: How many digits of Pi do you know?

I once memorized a deck of cards on one such day. That is equal to about 68 digits of PI. That's not impressive. I've memorized a deck of cards, myself. Wait...did you mean in order? Last time i had a cold, i taught myself ASL finger-spelling using this site . Not sure how much more useful that is ...