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seen Apr 21 at 14:44

Mar
26
comment Normal distribution percentile calculation
Intuitive answer: 97.8% (less than 99%) of the parcels will be less than 2 deviations above the mean. 2 deviations above the mean is 19 pounds, so 16.465 can't possibly be right (even excluding the extra pound). Incidentally, this model isn't very good because it states that some packages will weigh less than 0 pounds.
Mar
26
comment love in an elevator
Possibly simpler: combined weight of 16 people averages 1200kg with a standard deviation of 40kg (variance increases linearly with the number of people, so standard deviation increases with the square root of the number of people). As noted above, the chance this exceeds 1140kg is the chance the standard normal variable exceeds -1.5 standard deviations, or about 0.9332 as noted above
Mar
26
comment love in an elevator
@JoelReyesNoche What's love but a second-hand emotion?
Mar
26
comment What is the cartesian equation of $r = 1 + r \sin(\theta)?$
Well, r^2 = x^2 + y^2 and tan(theta) = y/x, and go from there.
Mar
13
comment Population average age decreases with births AND deaths (kind of)?
Of course! As long as the age decrease I describe is slower than the flow of time, the average age still increases. I will approve this answer as soon as stack lets me.
Feb
23
comment Calculating a Point that lies on an Ellipse given an Angle
@Atraxia this may help: math.stackexchange.com/questions/493104/…
Feb
23
comment Ellipse in polar coordinates
You may also want to look at my answer to math.stackexchange.com/questions/493104/…
Feb
20
comment Explain $\iint \mathrm dx\mathrm dy = \iint r \mathrm d\alpha\mathrm dr$
Why not take the area of the entire triangle (r^2/2*dtheta) and integrate over theta? ;)
Feb
1
comment Why is this coin-flipping probability problem unsolved?
What if you know in advance the coin will come up heads p% of the time, even for p!=50? That's sort of the question I asked at: quant.stackexchange.com/questions/2172/… (and no answers there yet either)
Dec
10
comment Best fit circle to “planetary” elliptical orbit?
@hardmath I'd be interested in seeing a solution like that, but the idea of using a circle is to make the math easy. If the nonuniform motion were ugly enough, it would defeat the purpose. However, I'll upvote (but not approve) an answer like that, just to see what it looks like.
Dec
10
comment Best fit circle to “planetary” elliptical orbit?
@RahulNarain OK, I'm willing to do that (ie, accept an answer that minimizes d^2, not d). It seems traditional to minimize the distance squared, even though it's different from minimizing the distance itself.
Dec
1
comment Can an algorithm be part of a proof?
The proof of the Four-Color Theorem is probably the most famous example of proof-by-algorithm.
Nov
30
comment Show equation has at most two solutions on (0,2*Pi)
Thanks, edited. I want this for ANY function dec[ha] (that satisfies the given conditions).
Nov
13
comment Stumped by Common Core math problem
I still don't see how this is a rotation. Around which point do we rotate WXY to get CBA?
Nov
13
comment Stumped by Common Core math problem
Thanks! @JoelReyesNoche I think your answer was equivalent, but this answer is more detailed, and I understood it better, so giving the checkmark here.
Nov
13
comment Stumped by Common Core math problem
Unfortunately, I don't have the answer (I'm actually posting on behalf of someone else).
Sep
7
comment Map Earth surface so straight line distance is great circle distance?
OK, I see it now. Because the distance from the North Pole to the equator must be EXACTLY 1/2 the distance from the North Pole to the South Pole, the entire equator must map to one point, which obviously doesn't work. This also shows you can't do it within a specified tolerance.
Sep
7
comment Map Earth surface so straight line distance is great circle distance?
More symbols, please. Or, better still, could you pick 3-4 specific points and show why this would be impossible? I think the proof you give works even for the straight line distance between two points on Earth, and it's obviously possible to map those in R^3
Sep
7
comment Map Earth surface so straight line distance is great circle distance?
OK, but doesn't the equator just have to map to a set of points equidistant from the poles? It doesn't have to map to the midpoint of the poles?
Sep
7
comment Map Earth surface so straight line distance is great circle distance?
First answer: but what about the Earth in 3-D. Every point on the equator is equidistant from the poles, no? In 2D this would be the perpendicular bisector. In 3D, it's a plane. In n dimensions, it's an n-1 dimensional surface, no? Second answer: what if I wanted to specify the tolerance. Is there always an 'n' that satisfies for a given tolerance?