#194379 - 20/12/2003 08:10
Maths question.
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pooh-bah
Registered: 14/01/2002
Posts: 2489
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My dad is fitting a new kitchen worktop to fit around a new cooker.
There is an arc-shape to it, but he needs to know the radius of the circle that will result from the measurements in the diagram. Measurement from the tip of the arc to the theoretical straight edge of unit is 20.
Unit is 600 in width.
Anyone know?
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#194380 - 20/12/2003 08:51
Re: Maths question.
[Re: CrackersMcCheese]
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pooh-bah
Registered: 14/01/2002
Posts: 2489
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#194381 - 20/12/2003 09:55
Re: Maths question.
[Re: CrackersMcCheese]
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carpal tunnel
Registered: 29/08/2000
Posts: 14493
Loc: Canada
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I visited a geocache once that required solving this same exact problem.
Cheers
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#194382 - 20/12/2003 09:58
Re: Maths question.
[Re: mlord]
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pooh-bah
Registered: 14/01/2002
Posts: 2489
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#194383 - 20/12/2003 09:59
Re: Maths question.
[Re: CrackersMcCheese]
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carpal tunnel
Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
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Since it's an interesting problem anyway, let's see:
c = 2 * sqrt(h * (2r - h))
c = 600
h = 20
600 = 2 * sqrt(20 * (2r - 20))
300 = sqrt(20 * (2r - 20))
90000 = 20 * (2r - 20)
4500 = 2r - 20
4520 = 2r
2260 = r
So the radius is 2260. Is that what you got?
_________________________
Bitt Faulk
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#194384 - 20/12/2003 10:05
Re: Maths question.
[Re: wfaulk]
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pooh-bah
Registered: 14/01/2002
Posts: 2489
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#194385 - 20/12/2003 10:08
Re: Maths question.
[Re: CrackersMcCheese]
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carpal tunnel
Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
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I wish I'd found some real trigonometry instead of just a straight formula for the answer, though.
_________________________
Bitt Faulk
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#194386 - 20/12/2003 10:11
Re: Maths question.
[Re: wfaulk]
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pooh-bah
Registered: 14/01/2002
Posts: 2489
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Yeah, theres a strange sense of pride and accomplishment when you actually use your brain to problem solve. My dads just happy to have the answer though!
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#194387 - 20/12/2003 10:33
Re: Maths question.
[Re: wfaulk]
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carpal tunnel
Registered: 29/08/2000
Posts: 14493
Loc: Canada
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Well, speaking of tricky problems.. perhaps one of you can help with this one:
You are out in a high grass, sparsely-treed, level wilderness, on an overcast summer's day, on a very important quest looking for a bucket of trinkets of little value. Suddenly you startle a momma bear with cubs!!! HOLY MOLEY!!! She immediately attacks you! WHAM!!! WHAM!!!WHAM!!! Using current wisdom, you play dead, praying that it's not a rehearsal for the real thing. After an eternity of her beating and mauling you, she and her cubs disappear.
When you come to, all is quiet except your heart which is pounding like a mad drummer's tattoo. Bleeding profusely, you decide to clear out of there in a HURRY! You check your GPSr and find that it too has taken a beating. It works, but all your waypoints are gone and so is your tracklog. The only thing you are certain of, is that just before you were attacked, you were beside that tree right there which is EXACTLY 1 kilometer from a perfectly STRAIGHT trail which will lead you to safety. You have no idea in which direction to start off. Fighting the very strong temptation to head off in all directions at once, you force yourself to calm down. You realize that you must get out as soon as possible, or bleed to death. The adrenelin coursing through your body makes your brain work at the speed of light and your thoughts become crystal clear.
You immediately devise an OPTIMAL geometric plan that will guarantee that you find the trail in the SHORTEST travelling distance from that tree compared to ANY other plan, even if you start off using the WORST possible choice of heading. There is no sun, or anything else, to guide you as to direction. You also know you won't see the trail until you are right on top of it.
As it happens, you DO choose the worst initial heading in following the optimal strategy, but your plan works and you manage to get to help just in time to save your life.
Looks like a mild calculus problem to me, but it's been over 20 years since I last did *any* calculus.
Cheers
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#194388 - 20/12/2003 12:43
Re: Maths question.
[Re: wfaulk]
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pooh-bah
Registered: 25/08/2000
Posts: 2413
Loc: NH USA
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Browsing that Wolfram site Bitt linked to makes my head spin, and in my day I was pretty good at math (AP Calculus in 9th grade etc...). I'm getting old. 
-Zeke
_________________________
WWFSMD?
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#194389 - 20/12/2003 15:58
Re: Maths question.
[Re: mlord]
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Anonymous
Unregistered
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Alright, I'm going to say walk 1 km in any direction. Then walk in a circle that has a radius of 1 km with the tree as the center. That's a total of 7.283 km.
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#194390 - 20/12/2003 17:22
Re: Maths question.
[Re: ]
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carpal tunnel
Registered: 29/08/2000
Posts: 14493
Loc: Canada
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Simple, but definitely non-optimal.
how to do it with less walking in the worst case?
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#194391 - 20/12/2003 17:42
Re: Maths question.
[Re: mlord]
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pooh-bah
Registered: 31/08/1999
Posts: 1649
Loc: San Carlos, CA
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how to do it with less walking in the worst case?
Walk the square root of 2 (approx 1.4) kilometers starting from the tree in each of 4 directions where each direction is 90 degrees from the previous direction (i.e. north, south, east, west). This should cause you to cross any straight trail coming within a mile of your current position and worst case would be walking approx 5.7 miles before finding the trail. I think?
-Mike
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#194392 - 20/12/2003 17:47
Re: Maths question.
[Re: mcomb]
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carpal tunnel
Registered: 29/08/2000
Posts: 14493
Loc: Canada
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But would that not require backtracking to the center each time, adding to the mileage?
Cheers
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#194393 - 20/12/2003 19:06
Re: Maths question.
[Re: mlord]
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pooh-bah
Registered: 31/08/1999
Posts: 1649
Loc: San Carlos, CA
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But would that not require backtracking to the center each time, adding to the mileage?
Doh! Ah, hell, I give up. In reality I am busy running in the opposite direction the bear went anyway 
-Mike
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#194394 - 20/12/2003 21:18
Re: Maths question.
[Re: mlord]
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enthusiast
Registered: 19/04/2001
Posts: 369
Loc: Seattle, WA (formerly Houston,...
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definitely non-optimal. I like to think I'm (usually!) pretty good at these, and I don't see how [censored]'s answer is not optimal, given how I understand the problem.
If that's definitely not optimal, could you explain a counter example for us? Maybe it will help us understand the problem in a different way.
John
_________________________
1998 BMW ///M3
30 GB Mk2a, Tuner,
and 10 GB backup
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#194395 - 20/12/2003 21:43
Re: Maths question.
[Re: johnmcd3]
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carpal tunnel
Registered: 29/08/2000
Posts: 14493
Loc: Canada
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I don't actually know the solution (thus my posting here), but the case of a straight radius line plus a full circle is obvious -- a 3/4 circle is just as good as a full circle and with less distance. But is there an even better solution?
Cheers
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#194396 - 20/12/2003 22:48
Re: Maths question.
[Re: mlord]
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veteran
Registered: 21/01/2002
Posts: 1380
Loc: Erie, CO
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1.4142 = square root of 2
Walk 1.4142km from the tree. Turn 45 degrees left, then walk 1km.
Follow an arc with the radius of 1km from the tree for 3.1416km
Walk 1km straight from the point you stopped.
This is a horseshoe shaped path.
Distance = 6.556km
This path will intersect all tangents of the circle (worst case what we are looking for, I'm guessing). That's as good as I can do! Please post the real solution when you find it, Mark!
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#194397 - 21/12/2003 00:25
Re: Maths question.
[Re: cushman]
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Anonymous
Unregistered
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Well I'll be damned, you're right. Nice job. I drew a little picture of it for everyone to see (a little messy)
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#194398 - 21/12/2003 00:34
Re: Maths question.
[Re: ]
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Anonymous
Unregistered
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now what if you divided it up into thirds. Would this be an even shorter path? He would only have to walk one third of the circle, but he'd have to go further out than .414 km.
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#194399 - 21/12/2003 01:04
Re: Maths question.
[Re: ]
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Anonymous
Unregistered
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negative. I got 7.558 km. So cushman's answer appears to be the shortest
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#194400 - 21/12/2003 01:37
Re: Maths question.
[Re: ]
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Anonymous
Unregistered
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One last proposition. Cushman's answer was based on dividing the circle into four equal arcs, or drawing a square around the circle:
My last answer was based on dividing the circle into 3 parts, and the path was longer than Cushman's. Dividing it into 2 parts wouldn't be possible, he would just keep walking out to infinity never reaching the point where he needs to make his turn. so...
2= infinity
3= 7.55
4= 6.55
so what would happen if you drew a pentagon around the circle.
he would walk out from the tree for 1 km plus a short distance smaller than .414 km. then he'd turn, meet the circle and walk four fifths of it, then walk another short distance off on a tangent.
I'm too tired to do any more math, but I doubt it will be shorter than 6.55. it will probably just get higher and higher for inifinity the more you divide up the circle but never reaching the circumference of the circle added to the radius. In fact, now I'm positive it will do that without doing the math. I think cush's is the shortest possible. Good job, cushman.
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#194401 - 21/12/2003 01:56
Re: Maths question.
[Re: ]
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Anonymous
Unregistered
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fuck it. The shapes are irrelavent. rather, any fraction of the circle can be traversed with going out on a tangent some to cover the parts of the circle you missed. I assume 3/4 of the circle is the peak where the path is the shortest. i reckon you need calculus, which i don't know, to find the magic number. I guess that's how you got the answer, cush, rather than using these gay shapes with geometry and trigonometry.
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#194402 - 21/12/2003 02:36
Re: Maths question.
[Re: cushman]
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pooh-bah
Registered: 31/08/1999
Posts: 1649
Loc: San Carlos, CA
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This is a horseshoe shaped path.
A ha! You know that is kind of what I was going for (pythagorean theorem to determine the farthest you would have to walk to intersect the path of a straight line touching the circle), I just F'd up what to do once you get there
-Mike
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#194403 - 21/12/2003 08:10
Re: Maths question.
[Re: ]
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veteran
Registered: 21/01/2002
Posts: 1380
Loc: Erie, CO
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Nope, I just used intuition, but it could be done with trig and a graph fairly easily. We are searching for the optimal angle a that will yield the shortest distance of line e and arc g. The tough part is the distance of line e. You would have to build an equation to find the distance of line d (trig), then an equation to determine e. Angles c and b = 90 degrees. Arg g can be determined with an equation to calculate angle f.
I haven't done all of this since I believe the best angle would be 90 degrees, but it could be proven.
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#194404 - 21/12/2003 08:47
Re: Maths question.
[Re: cushman]
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veteran
Registered: 21/01/2002
Posts: 1380
Loc: Erie, CO
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Oh, I forgot to add in the distance to one of the staring points of the horseshoe. That would add in one more section for the equation (but still doesn't change my answer).
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#194405 - 21/12/2003 14:35
Re: Maths question.
[Re: mlord]
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carpal tunnel
Registered: 30/04/2000
Posts: 3810
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Going back and re-reading the original problem, there's no constraint on the direction of the "perfectly straight" path. The path could be tangential to the 1km circle or it could point radially outward. Given that, here's the worst case. Imagine you walk due north to the 1km circle, stop, turn 90 degrees left, and start tracing out the circle. Imagine the path actually started just to your right and had a heading toward the left, just a few degrees above being tangential with the circle. In this case, any divergence you make from the circle until you get all the way around will guarantee that you never interesect with the path to safety.
If you're trying to minimize your worst-case walking distance, then the simple answer (walk the full circle) seems to be the right answer. However, if we change the question around to be minimizing your *expected* case walking distance, given an equal probability of the safe road being at any starting point in any direction, then things get more interesting. The concern, if you stick on the 1km circle, is that you don't get to "cut off the corner" by intersecting with the safe path somewhere beyond the 1km radius.
I suspect that a useful strategy would be some kind of increasing radius arc. You need to solve for d radius / d theta, such that the expected length of the walk is minimized. To compute the expectation, you've got to integrate over all possible paths. You also have to know how long the path is that takes you to safety, such that you don't overshoot wherever that safe path takes you. I wonder, however, whether you'd just end up with d radius / d theta = 0, getting you back to the degenerate, simple solution. (Also, since the length of the safe path isn't specified, I'll bet the problem's author never considered this sort of solution.)
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#194406 - 21/12/2003 19:05
Re: Maths question.
[Re: DWallach]
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carpal tunnel
Registered: 29/08/2000
Posts: 14493
Loc: Canada
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In this case, we do know that the *minimum* distance to the perfectly straight path is 1KM -- so no worries about obfuscation there. I will test the solution sometime in a week and a bit.
Thanks all!
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#194408 - 22/12/2003 04:41
Re: Maths question.
[Re: lectric]
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pooh-bah
Registered: 14/01/2002
Posts: 2489
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LOL 
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