Wikipedia:Reference desk/Archives/Mathematics/2007 August 16

= August 16 =

baseball and Math
well i ma looking for information about math reffering wat u can do with math in baseball can u help —The preceding unsigned comment was added by Special:Contributions/ (talk)


 * Sabermetrics may be of interest. PrimeHunter 01:10, 16 August 2007 (UTC)


 * At a more basic level, fundamental statistics like batting average and earned run average are mathematical expressions. &mdash; Lomn 15:56, 16 August 2007 (UTC)

I've always found Simpson's_paradox to be an interesting case. StuRat 07:25, 18 August 2007 (UTC)

Standard deviation
Alright guys, here's a simple one. If I know the standard deviations of two independent random variables, how do I calculate the standard deviation of their difference? 151.152.101.44 17:50, 16 August 2007 (UTC)


 * Since they're independent, you could take the square root of the sum (after edit conflict: yes, thank you Meni) of the squares of the two known standard deviations. Variance has more details (under "Variance of a weighted sum of variables"). iames 18:49, 16 August 2007 (UTC)


 * Actually, you need the sum, not the difference. -- Meni Rosenfeld (talk) 19:00, 16 August 2007 (UTC)

Trigonometry & Complex Numbers
How we can obtain Sin(A+iB) or Sinh(A+iB). (Smart_Viral 21:19, 16 August 2007 (UTC))


 * sin(a+ib) = sin(a)*cosh(b) + cos(a)*sinh(b)i


 * sinh(a+ib) = sinh(a)*cos(b) + cosh(a)*sin(b)i


 * I love my Voyage 200... hope that helps! Gscshoyru 21:27, 16 August 2007 (UTC)

Thanks ;) Smart_Viral 21:45, 16 August 2007 (UTC)

Body fat calculation
I am a complete idiot, please help me out. Let's say I weigh 200lbs and 15% of my bodyweight is fat. Thus, my fat-free mass is 170lbs. If I want to calculate how much I would weigh at 8% body fat, how do I calculate? I've tried 170*1.08 and 170/.92, but neither seems to be correct, yet I can't think of a different way to calculate it. Please help me! Jack Daw 22:33, 16 August 2007 (UTC)
 * Multiply the amount of fat (30 lbs) by 8/15, and add that back to your non-fat mass.Strad 22:59, 16 August 2007 (UTC)
 * 170/.92 is right. Multiply it back out: 184.8·.92 = 170.0. Strad 23:08, 16 August 2007 (UTC)


 * Hmm ok. The thing that bugs me is why 1 percentage unit leads to a different number as you go higher in BF%. For example: 170/.99-170 = 1.72; 200-170/.86 = 2.33. Why? Jack Daw 00:59, 17 August 2007 (UTC)


 * You compare 1% of different total weights. If your fat increases then your total weight increases and 1% of a larger weight is more. PrimeHunter 01:44, 17 August 2007 (UTC)


 * Yes, this is a common problem people have with percentages, when the base changes. The base for 15% was 200 lbs, while the base for the 8% is 184.8 lbs (which you didn't know, at first).  Also, I should point out that if you lose 15.2 lbs, it's unlikely that all of that weight lost will be fat, as any diet severe enough to cause such a weigh loss will likely cause you to lose other forms of weight (such as water), as well. StuRat 07:19, 18 August 2007 (UTC)


 * Yes, percentages do give some people probems when this happens. It's a bit like a problem I heard involving a sack of potatoes.
 * The potatoes have a mass of 100kg, but normally 99% of that is just water. One this particular day, the potatoes have been left out in the blazing sun, and have de-hydrated so that now 98% of the mass is water. What is the mass of the sack of potatoes? Of course, most people expect it to be similar to the original mass, whereas in fact, the mass has halved to only 50kg. Richard B 22:28, 18 August 2007 (UTC)