Hey there! Sign in to join this conversationNew here? Join for free
 You are Here: Home >< Maths

# S1 discrete random variables Q help. Watch

1. http://www.edexcel.com/migrationdocu...e_20110114.pdf

Q6 e,f,g
for a, showed k =0.1
b. E(X)=3
c E(X2)=10
d. Var(2-5X)=25, as Var(X)=1
e. i know that to get a value of 4, you need to have a combination of (1,3) and (2,2), what should i do next? thanks
f. Stuck
g. Stuck

Thanks

+rep too
2. (Original post by Hi, How are you ?)
http://www.edexcel.com/migrationdocu...e_20110114.pdf

Q6 e,f,g
for a, showed k =0.1
b. E(X)=3
c E(X2)=10
d. Var(2-5X)=25, as Var(X)=1
e. i know that to get a value of 4, you need to have a combination of (1,3) and (2,2), what should i do next? thanks
f. Stuck
g. Stuck

Thanks

+rep too
For e, you need to consider all the different ways of making 4. You are correct in thinking you need combinations of (1,3) and (2,2). Using these "combinations", how many different ways can you add them to get the number 4?

EDIT: Then for 'f', you use a similar thing. Let me explain how they got one.

Let's look at how they got .

The only way you can make 2 = 1 + 1, as you can't get 2 + 0 (as you can't get 0). In other words, you want 1 AND 1, i.e P(X = 1) * P(X = 1) = 0.1 * 0.1 = 0.01.

Now, you use this similar method to get the rest.

EDIT: For 'g', you need to think things through a little. You want the probability of .

Now, you know quite clearly that you can't get probability P(X = something . 5). So you only look at integers. Does this make a little more sense in what to do?
3. (Original post by claret_n_blue)
For e, you need to consider all the different ways of making 4. You are correct in thinking you need combinations of (1,3) and (2,2). Using these "combinations", how many different ways can you add them to get the number 4?

EDIT: Then for 'f', you use a similar thing. Let me explain how they got one.

Let's look at how they got .

The only way you can make 2 = 1 + 1, as you can't get 2 + 0 (as you can't get 0). In other words, you want 1 AND 1, i.e P(X = 1) * P(X = 1) = 0.1 * 0.1 = 0.01.

Now, you use this similar method to get the rest.

EDIT: For 'g', you need to think things through a little. You want the probability of .

Now, you know quite clearly that you can't get probability P(X = something . 5). So you only look at integers. Does this make a little more sense in what to do?
hi, so for e, I done (0.1*0.3)+(0.2*0.2)=0.07, what did i do wrong?
4. (Original post by Hi, How are you ?)
hi, so for e, I done (0.1*0.3)+(0.2*0.2)=0.07, what did i do wrong?
There are two ways of getting 4 with 1 and 3.

You either get 1 first and then 3 OR 3 first and then 1.
5. (Original post by claret_n_blue)
There are two ways of getting 4 with 1 and 3.

You either get 1 first and then 3 OR 3 first and then 1.
Doesn't that mean they are 2 ways for getting (2,2), if you get what I mean, even though they are the same values?
6. (Original post by Hi, How are you ?)
Doesn't that mean they are 2 ways for getting (2,2), if you get what I mean, even though they are the same values?
No. Because they're the same value there's not two ways. If you think of it as children and let's say we are looking the different ways of either getting 1 boy and 1 girl or 2 boys. There are two ways of getting one boy and 1 girl. Either the boy is born first, then the girl, or the girl is born first and then the boy. However, there is only 1 way of getting the two boys, first one boy is born, then the other.

Similarly, there is only way of making 4 from 2 and 2 and that is simply 2 + 2 = 4. Order here doesn't matter as it will still be the same thing

With 1 and 3 however, either you get 1 first then 3 or 3 first then 1.
7. (Original post by claret_n_blue)
No. Because they're the same value there's not two ways. If you think of it as children and let's say we are looking the different ways of either getting 1 boy and 1 girl or 2 boys. There are two ways of getting one boy and 1 girl. Either the boy is born first, then the girl, or the girl is born first and then the boy. However, there is only 1 way of getting the two boys, first one boy is born, then the other.

Similarly, there is only way of making 4 from 2 and 2 and that is simply 2 + 2 = 4. Order here doesn't matter as it will still be the same thing

With 1 and 3 however, either you get 1 first then 3 or 3 first then 1.
I see, thanks, can't rep at the moment, sorry

Reply
Submit reply
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: April 10, 2013
Today on TSR

### Anxious about my Oxford offer

What should I do?

### Am I doomed because I messed up my mocks?

Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

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

• Poll
Useful resources

## Make your revision easier

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams

Can you help? Study help unanswered threads

## Groups associated with this forum:

View associated groups
Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

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

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.