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S1 formula booklet

Hello.

In the Edexcel formula booklet we get, what is the 3rd formula under the probability heading used for?
I have gone through my textbook but i cant seem to see it anywhere. I tried to derive it too, but no luck.

Ill attach a photo of it. Thanks. ImageUploadedByStudent Room1394041985.670093.jpg

ImageUploadedByStudent Room1394041985.670093.jpg

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Reply 1

davros

Original post by jtbteddy

Posted from TSR Mobile

It just gives you a way of working out conditional probability one way round when you know the conditional probabilities "the other way round" if you see what I mean.

If you recall the other formula involving conditional probabilities - P(AnB) = P(A|B) x P(B) then the numerator is just P(AnB) and the denominator is P(B) so it's basically relating the 2 ways of working out P(AnB) = P(BnA) :smile:

Reply 2

CTArsenal

It's just another way of writing P(A|B).

Conventionally P(A|B) = P(A∩B)/P(B); they've just turned it into another format involving the P(B|A). You rarely ever use that one so there's no need to be worried about that one haha.