# probability - the addition ruleWatch

#1
ok ... i get really confused with the addition rule. the exam's 2mro .. help anyone.

if i want P(A'nB) and only have P(A) and P(B) and P(AnB) how would i work this out?

how do i work out P(') using the addition rule

how would i work out the following using the addition rule?

P(A'nB)
P(AnB')
P(A'nB)

and

P(A'uB)
P(AuB')
P(A'uB')

thanks guys ~ Sabz
0
12 years ago
#2
(Original post by rename)
if i want P(A'nB) and only have P(A) and P(B) and P(AnB) how would i work this out?

P(A'nB)
P(AnB')
P(A'nB)

and

P(A'uB)
P(AuB')
P(A'uB')
P(A'nB) = P(B) - P(AnB)
p(AnB') = P(A) - P(AnB)

P(A'uB) = P(A') + P(B) - P(A'nB)
P(AuB') = P(A) + P(B') - P(AnB')
P(A'uB') = P(A') + P(B') - P(A'nB')

Use the fact that P(A') = 1-P(A), P(B') = 1 - P(B) and P(A'nB') + P(AuB) = 1 and you're done.
0
#3
(Original post by Widowmaker)
P(A'nB) = P(B) - P(AnB)
p(AnB') = P(A) - P(AnB)

P(A'uB) = P(A') + P(B) - P(A'nB)
P(AuB') = P(A) + P(B') - P(AnB')
P(A'uB') = P(A') + P(B') - P(A'nB')

Use the fact that P(A') = 1-P(A), P(B') = 1 - P(B) and P(A'nB') + P(AuB) = 1 and you're done.
thanks you're a star ... i'm just incredibly or rather very credibly stupid and slow when its comes to probability ... haha
0
12 years ago
#4
(Original post by Widowmaker)
P(A'nB) = P(B) - P(AnB)
p(AnB') = P(A) - P(AnB)

P(A'uB) = P(A') + P(B) - P(A'nB)
P(AuB') = P(A) + P(B') - P(AnB')
P(A'uB') = P(A') + P(B') - P(A'nB')

Use the fact that P(A') = 1-P(A), P(B') = 1 - P(B) and P(A'nB') + P(AuB) = 1 and you're done.

sorry could you explain how you came up with them using

P(A') = 1-P(A), P(B') = 1 - P(B) and P(A'nB') + P(AuB) = 1

i just dont see how you got them.
0
12 years ago
#5
(Original post by Bravery)
sorry could you explain how you came up with them using

P(A') = 1-P(A), P(B') = 1 - P(B) and P(A'nB') + P(AuB) = 1

i just dont see how you got them.
P(A) = probability of event A
P(A') = probability of not event A

If you have a particular event. E.g. I will throw a 6. The probability of you throwing a 6 or not throwing a 6 is 1. Hence P(A) + P(A') = 1

P(A'nB') = probability of not A and not B.
P(AuB) = probability of A or B or both A and B

This is similar to P(A) + P(A') = 1
=> P(A'nB') + P(AuB) = 1

If you draw a Venn diagram with a sample space and label A'nB' and AuB, you'll see why I'm right.
0
12 years ago
#6
sorry I didn't make myself very clear

Did you use a Venn Diagram to come up with:

P(A'nB) = P(B) - P(AnB)
p(AnB') = P(A) - P(AnB)

P(A'uB) = P(A') + P(B) - P(A'nB)
P(AuB') = P(A) + P(B') - P(AnB')
P(A'uB') = P(A') + P(B') - P(A'nB')

as i couldnt work out how you got them

thanks a lot.
0
12 years ago
#7
But this only applies to events that are not mutually exclusive...so for independent events...this wont be true?

Thanks
From John
0
12 years ago
#8
(Original post by Bravery)
sorry I didn't make myself very clear

Did you use a Venn Diagram to come up with:

P(A'nB) = P(B) - P(AnB)
p(AnB') = P(A) - P(AnB)

P(A'uB) = P(A') + P(B) - P(A'nB)
P(AuB') = P(A) + P(B') - P(AnB')
P(A'uB') = P(A') + P(B') - P(A'nB')

as i couldnt work out how you got them

thanks a lot.
I used a venn diagram yes.
Also used P(AuB) = P(A) + P(B) - P(AnB) which is a standard formula.
0
X

new posts
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### University open days

• Cranfield University
Cranfield Forensic MSc Programme Open Day Postgraduate
Thu, 25 Apr '19
• University of the Arts London
Open day: MA Footwear and MA Fashion Artefact Postgraduate
Thu, 25 Apr '19
• Cardiff Metropolitan University
Sat, 27 Apr '19

### Poll

Join the discussion

#### Have you registered to vote?

Yes! (228)
39.31%
No - but I will (40)
6.9%
No - I don't want to (41)
7.07%
No - I can't vote (<18, not in UK, etc) (271)
46.72%

View All
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.