theTruthshaper Posted March 3, 2021 Report Share Posted March 3, 2021 (edited) On 21/12/2020 at 11:12 AM, G2F4E6E7E8 said: Let f be a real-valued function on the plane such that for every square ABCD in the plane, f(A) + f(B) + f(C) + f(D) = 0. Does it follow that f(P) = 0 for all points P in the plane? Yes. For every point P, we can take points W X Y Z, and points A B C D as such: So, we have 4 squares, and adding the equations for each of those we get 4*f(P) + 2*( f(W) + f(X) + f(Y) + f(Z) ) + ( f(A) + f(B) + f(C) + f(D) ) = 0 Since the second term and the third term also represent squares, their sum is also 0. Thus, 4*f(P) = 0 => f(P) = 0 Hence, proved. On 7/2/2021 at 5:47 AM, AquaRegia said: Card probability problem from real life: my wife and I are playing a variant of Pinochle. 4 players, 48 card deck, 9-10-J-Q-K-A only in 4 suits, so two of each card. Each player holds 12 cards and bidding team pass 3 cards to each other before play begins. If I need one specific card from my partner (say, an ace of clubs), what is the probability they will have it? My thoughts: I already know the card I need is NOT in the 12 I have, so it must be in one of the other 3 hands. P=1/3 that it is in my partner's hand, yes? That means P=2/3 that it is in one of my opponents'. But there are TWO aces of clubs, and I only need one. So the only BAD outcome is if BOTH of them are in my opponents' hands. Pbad = (2/3)(2/3) = 4/9 = 44.4%, therefore there is a 1 - 4/9 = 55.6% chance my partner has at least one of the aces I need. Is this the correct solution? And how would I extend this work to cover the case where I need TWO cards (say, K and Q of clubs)? Is it just (5/9)(5/9) = 25/81? This isn't exactly right, because the probabilities of the second ace are not independent of the first. So, P(bad) = (2/3)*(23/35) = (46/105) ~ 0.438. Which implies that there is a 56.2% chance your partner has at least one of them. Similarly, when you need two different cards, P(bad) = 2*(2*23/3*35) - (2*23*22*21/3*35*34*33) ~ 0.696 Thus, there is a 30.4% chance that your partner has at least one of both of them. Edited March 3, 2021 by theTruthshaper 0 Quote Link to comment Share on other sites More sharing options...
AquaRegia Posted March 3, 2021 Report Share Posted March 3, 2021 14 hours ago, theTruthshaper said: Thus, there is a 30.4% chance that your partner has at least one of both of them. Thank you! Honestly I'm just happy that my initial estimates were both within 1%, as I've never taken a course in probability in my life. 0 Quote Link to comment Share on other sites More sharing options...
Silverblade5 Posted March 12, 2021 Report Share Posted March 12, 2021 Lets try this one on for size 50 - 2sin(x) * (3 + 2cos(x)) = 0 Solve for x 0 Quote Link to comment Share on other sites More sharing options...
theTruthshaper Posted March 12, 2021 Report Share Posted March 12, 2021 8 hours ago, Silverblade5 said: Lets try this one on for size 50 - 2sin(x) * (3 + 2cos(x)) = 0 Solve for x No such x. 0 Quote Link to comment Share on other sites More sharing options...
Dunkum Posted March 12, 2021 Report Share Posted March 12, 2021 19 minutes ago, theTruthshaper said: No such x. definitely not in the reals. maybe complex? I don't really know what values sin and cos can take with complex arguments. but yea for real x, sin and cos both max out at 1 so if you replace both of them with that then you get 50 -(2*(1)*(3+2(1))=50-(2*5)=40-10 which is obviously not 0. and of course cos(x) and sin(x) can't both be 1 at the same time, but ultimately that doesn't matter. 0 Quote Link to comment Share on other sites More sharing options...
Silverblade5 Posted March 13, 2021 Report Share Posted March 13, 2021 On 3/12/2021 at 7:18 AM, Dunkum said: definitely not in the reals. maybe complex? I don't really know what values sin and cos can take with complex arguments. but yea for real x, sin and cos both max out at 1 so if you replace both of them with that then you get 50 -(2*(1)*(3+2(1))=50-(2*5)=40-10 which is obviously not 0. and of course cos(x) and sin(x) can't both be 1 at the same time, but ultimately that doesn't matter. Fair enough lol. To make it possible, change 2sin(x) to 2tan(x), as tan(x) has an infinite real range. Alternatively, Solve e^x = x for x. 0 Quote Link to comment Share on other sites More sharing options...
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