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 Feb 21 awarded Notable Question Nov 24 revised Relationship between two random, normally distributed variables added 11 characters in body May 21 awarded Popular Question Sep 1 awarded Autobiographer Jan 15 accepted Relationship between two random, normally distributed variables Jan 15 comment Relationship between two random, normally distributed variables I'm not discounting the possibility that my book is wrong; it has an awful lot of errors... If you stand by your latest answer? Jan 15 comment Relationship between two random, normally distributed variables Well, I'm starting to feel stupid here. We've been taught to consider $$D = aX + bY$$ which in this case should be $$D = -X + 0.7Y$$, but then I still get $$2ab\rho\sigma\sigma = 2 \cdot 0.7 \cdot -1 \cdot 0.8 \cdot 6 \cdot 7$$ which is negative? Jan 15 comment Relationship between two random, normally distributed variables Hm. The formula I have is $$D = aX + bY$$ and I used a = -1, b = 0.7. Then (from my book), $$\sigma^2_D = a^2\sigma^2_X + b^2\sigma^2_Y + 2ab\rho\sigma_X\sigma_Y$$ Jan 15 comment Relationship between two random, normally distributed variables I see... but I don't see why. The formula I have to find the variance of D does not use squares. It's just $$a^2\sigma^2 + b^2\sigma^2 + 2abCorr(X, Y)\sigma\sigma$$ (I'm being sloppy with the subscripts here, but yeah). Jan 15 asked Relationship between two random, normally distributed variables Nov 6 comment Creating pairs of homo-, bi- and heterosexual boys and girls I initially misinterpreted "if there are d bisexuals" to mean "exactly d". Thanks for the answer, it works smashingly! Nov 5 awarded Scholar Nov 5 accepted Creating pairs of homo-, bi- and heterosexual boys and girls Nov 4 awarded Supporter Nov 4 awarded Editor Nov 4 revised Creating pairs of homo-, bi- and heterosexual boys and girls added 4 characters in body Nov 4 awarded Student Nov 4 comment Creating pairs of homo-, bi- and heterosexual boys and girls @Phira: The man should, however, also like to marry her (i.e. he should not be homosexual). Nov 4 comment Creating pairs of homo-, bi- and heterosexual boys and girls @Phira: Indeed she would be! I'm kind of unfamiliar with this whole area of mathematics, but I'll have a look at the Wiki page :) Nov 4 asked Creating pairs of homo-, bi- and heterosexual boys and girls