S1 Maths

How to write using notation

Probability of getting A AND not B
Probability of getting A OR not B

what is the difference? I dont get it :/

Reply 1

VaVe

P(A n B')
P(A u B')

one means that it's the probability of being A and not being B
the other is the probability of something being A or not being B. There's only really a difference if there is a chance that something isn't A or B

p(AnB')
p(AuB')

?

You might find Swayum's posts in this thread helpful: http://www.thestudentroom.co.uk/showthread.php?t=1603015

(in particular, post #5)

Reply 4

jamesrune75

Original post by tehforum

p(AnB')
p(AuB')

?

I understand that, I dont understand what each represents on a VEnn diagram (wat is the difference between the two)

Reply 5

jamesrune75

Original post by VaVe

I don't get it... ( the last part of your quote)

????

Original post by jamesrune75

I don't get it... ( the last part of your quote)

my quote or my post?
I think the link EEngWillow posted probably explains it best.

Reply 8

Helsy

Original post by jamesrune75

I understand that, I dont understand what each represents on a VEnn diagram (wat is the difference between the two)

The first one P(AnB') means that everything that includes A and doesn't include B must be there. So for a venn diagram that would be just the A part, without the cross-over into the b circle.

The second one P(AuB') means that it must either be A or not be B. So that includes the whole circle of A (including the bit that goes into B) and then everything that isn't B (including the A ring and everything that is outside both rings).

I think that's right, people can correct me, I've not done S1 in ages.

Anyway, hope that helps. :smile:

Reply 9

gtmad

P(AnB') is everything which is BOTH in A and not in B
P(AuB') is everything which is EITHER in A or not in B

Reply 10

jamesrune75

I still don't understand :/ I hate situations like this where its hard to get your head round a simple concept//

Reply 11

nuodai

Let's say you're given a number at random.

Let A be the event that the number is even.
Let B be the event that the number is divisible by 3, so that "not B" means that the number isn't divisible by 3.

"A and not B" means that the number is even and can't be divided by 3, so for example 2 or 4, but not 5 since 5 isn't even, and not 6 since 6 is divisible by 3.

"A or not B" means that the number is even or that it can't be divided by 3. In this case, 5 satisfies it, because it can't be divided by 3; and 6 satisfies it, because it's even. [But 9 doesn't, because it's not even and it can be divided by 3.]

Does this make a bit more sense?

"A and not B" means that A has to be true and B has to be false. "A or not B" means that either A can be true, or B can be false (or both). So if A and B were both true then "A and not B" would be false, but "A or not B" would be true.

(edited 13 years ago)

Reply 12

VaVe

It's fine I hated S1, (and C1 and C2) .
I made a picture to help:
http://vaveblog.files.wordpress.com/2011/04/venn.jpg

Reply 13

jamesrune75

Original post by vave

it's fine i hated s1, (and c1 and c2) .
I made a picture to help:
http://vaveblog.files.wordpress.com/2011/04/venn.jpg

thanks! Spot on!!!!!!!

Reply 14

jamesrune75

Original post by nuodai

let's say you're given a number at random.

Let a be the event that the number is even.
Let b be the event that the number is divisible by 3, so that "not b" means that the number isn't divisible by 3.

"a and not b" means that the number is even and can't be divided by 3, so for example 2 or 4, but not 5 since 5 isn't even, and not 6 since 6 is divisible by 3.

"a or not b" means that the number is even or that it can't be divided by 3. In this case, 5 satisfies it, because it can't be divided by 3; and 6 satisfies it, because it's even. [but 9 doesn't, because it's not even and it can be divided by 3.]

does this make a bit more sense?

"a and not b" means that a has to be true and b has to be false. "a or not b" means that either a can be true, or b can be false (or both). So if a and b were both true then "a and not b" would be false, but "a or not b" would be true.