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# A level Maths- Stats help- formulae needed for probability watch

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1. Hi,
I am getting really frustrated by the edexcel book for the new spec as they have not given all the formulae needed to calculate probabilities.

Please can somebody give me the formula to work out=

P(A'|B) and P(A|B') and P(A'|B')

would really appreciate it thank you.
Hi,
I am getting really frustrated by the edexcel book for the new spec as they have not given all the formulae needed to calculate probabilities.

Please can somebody give me the formula to work out=

P(A'|B) and P(A|B' and P(A'|B'

would really appreciate it thank you.

Because B can be thought of as the entire sample space, and you're finding how much of the event A lies within in the event B in a venn diagram.
3. (Original post by NotNotBatman)

Because B can be thought of as the entire sample space, and you're finding how much of the event A lies within in the event B in a venn diagram.
Thanks for your help however I can work out simple A|B problems but I cant seem to do A'|B or A'|B', do i need to draw a venn diagram to help me?
Thanks for your help however I can work out simple A|B problems but I cant seem to do A'|B or A'|B', do i need to draw a venn diagram to help me?
It's the same formula, just change the events. So

can you see that I just replaced A with A' in the formula?

Venn diagrams are a very good way to visualise it. I would read P(A'|B) as "the probability of A not happening given B does happen". From there I would consider only the area of B (treating it as the sample space) on the venn diagram and see how much of it doesn't contain A.

Practise some questions and it will become very easy when you use a venn diagram, as it will become more clearer and won't feel like you're memorising formulas for no reason.
5. (Original post by NotNotBatman)
It's the same formula, just change the events. So

can you see that I just replaced A with A' in the formula?

Venn diagrams are a very good way to visualise it. I would read P(A'|B) as "the probability of A not happening given B does happen". From there I would consider only the area of B (treating it as the sample space) on the venn diagram and see how much of it doesn't contain A.

Practise some questions and it will become very easy when you use a venn diagram, as it will become more clearer and won't feel like you're memorising formulas for no reason.
thats a big help! Thank you very much

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Updated: October 11, 2017
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