Say I want to find Pr(A|B). Usually I would just use bayes rule, but some textbooks just assume that A|B works and then just multiply it by P(B). For example in my book, Pr(X>2000)=.4. X and Y are unif distributed from 1000 to 5000. They are independent. Find Pr(X+Y>8000|X>2000). Well they just said, ok look at the region bounded by X+Y>8000 and and X=2000 and Y=1000, Y=5000, X=5000. They found the area of the triangle and divided it by the region formed with X=2000, 5000, Y=1000, 5000.(adsbygoogle = window.adsbygoogle || []).push({});

Finally they multiplied it by .4.

I don't know what formula they use but obviously it is not Bayes rule.

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Conditional Probability without Bayes rule

**Physics Forums | Science Articles, Homework Help, Discussion**