Turn on thread page Beta
    • Thread Starter
    Offline

    2
    ReputationRep:
    Why does P(1.3≤X1 + X2≤3.2) round to p(2≤X1+X2<3) surely it would be P(1≤X1+X2≤3)
    Offline

    13
    ReputationRep:
    (Original post by creativebuzz)
    Why does P(1.3≤X1 + X2≤3.2) round to p(2≤X1+X2<3) surely it would be P(1≤X1+X2≤3)
    Well, if X1 and X2 are discrete random variables...
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by BuryMathsTutor)
    Well, if X1 and X2 are discrete random variables...

    Erm, could you elaborate on that please?
    Offline

    13
    ReputationRep:
    (Original post by creativebuzz)
    Erm, could you elaborate on that please?
    Could I see the whole question please?
    • Thread Starter
    Offline

    2
    ReputationRep:
    Attachment 461381461383
    (Original post by BuryMathsTutor)
    Could I see the whole question please?
    Oh yes of course, sorry! (two attachments)
    Name:  tumblr_mls07tVxpg1qd7o7ao1_500.jpg
Views: 68
Size:  18.3 KB
    Attached Images
     
    Offline

    13
    ReputationRep:
    (Original post by creativebuzz)
    Attachment 461381461383

    Oh yes of course, sorry! (two attachments)
    Name:  tumblr_mls07tVxpg1qd7o7ao1_500.jpg
Views: 68
Size:  18.3 KB
    As I expected, X is a discrete random variable. This means, in this case, X takes only certain integer values. X1+X2 is also an integer. If X1+X2 \ge 2.3 for example then in fact X1+X2 \ge 3.
    Offline

    1
    (Original post by BuryMathsTutor)
    As I expected, X is a discrete random variable. This means, in this case, X takes only certain integer values. X1+X2 is also an integer. If X1+X2 \ge 2.3 for example then in fact X1+X2 \ge 3.
    Do you take all 3 maths exams in the summer??
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by BuryMathsTutor)
    As I expected, X is a discrete random variable. This means, in this case, X takes only certain integer values. X1+X2 is also an integer. If X1+X2 \ge 2.3 for example then in fact X1+X2 \ge 3.

    But why is 1.3 rounded UP to 2 yet 3.2 is rounded DOWN to 3?
    Offline

    13
    ReputationRep:
    (Original post by creativebuzz)
    But why is 1.3 rounded UP to 2 yet 3.2 is rounded DOWN to 3?
    If X1+X2 \le 3.2 then X1+X2&lt;4 and so X1+X2 \le 3
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by BuryMathsTutor)
    If X1+X2 \le 3.2 then X1+X2&lt;4 and so X1+X2 \le 3
    I just don't understand why we are rounding up for 1.3 but rounding down for 3.2?
    • Study Helper
    Offline

    16
    ReputationRep:
    Study Helper
    (Original post by creativebuzz)
    I just don't understand why we are rounding up for 1.3 but rounding down for 3.2?
    Think about what values X1 + X2 is allowed to take!

    If X1 + X2 must be a whole number, then if X1 + X2 >= 1.3 we must have X1 + X2 >= 2. It isn't possible for it to be as small as 1.

    If X1 + X2 <= 3.2 then we know that X1 + X2 <= 3. It isn't possible for it to be as big as 4.

    It's the fact that X1 + X2 must be an integer that is crucial here.
    Offline

    2
    ReputationRep:
    (Original post by creativebuzz)
    I just don't understand why we are rounding up for 1.3 but rounding down for 3.2?
    It's because X1 + X2 has to be greater than 1.3, but it's gotta be less than 3.2. If you rounded 1.3 down, you'd be including values between 1 and 1.3, which we can't include as they don't fit within our inequalities. So we round up, because everything greater than 2 is also greater than 1.3. Similarly, we can't round 3.2 up cause then you'd include values between 3.2 and 4, which don't satisfy our inequality. So we round down, because everything less than 3 is also less than 3.2.

    Hope that makes sense.
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by davros)
    Think about what values X1 + X2 is allowed to take!

    If X1 + X2 must be a whole number, then if X1 + X2 >= 1.3 we must have X1 + X2 >= 2. It isn't possible for it to be as small as 1.

    If X1 + X2 <= 3.2 then we know that X1 + X2 <= 3. It isn't possible for it to be as big as 4.

    It's the fact that X1 + X2 must be an integer that is crucial here.
    Got it now, thank you!

    I know how to do this question now but my first attempt was to do:

    Not winning first:
    =1-0.5
    =0.5

    Not winning 2nd:
    1-0.3
    =0.7

    Therefore not winning first and second is 0.7 x 0.5 = 0.35

    Even though that answer is wrong, can you explain why my method was incorrect? (Just so I don't make the same mistake in the future)
    • Study Helper
    Offline

    16
    ReputationRep:
    Study Helper
    (Original post by creativebuzz)
    Got it now, thank you!

    I know how to do this question now but my first attempt was to do:

    Not winning first:
    =1-0.5
    =0.5

    Not winning 2nd:
    1-0.3
    =0.7

    Therefore not winning first and second is 0.7 x 0.5 = 0.35

    Even though that answer is wrong, can you explain why my method was incorrect? (Just so I don't make the same mistake in the future)
    Can you post the actual question so I can check the wording? This doesn't seem to relate to the question you've already posted in this thread
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: September 16, 2015
Poll
Cats or dogs?
Useful resources

Make your revision easier

Maths

Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

Equations

How to use LaTex

Writing equations the easy way

Student revising

Study habits of A* students

Top tips from students who have already aced their exams

Study Planner

Create your own Study Planner

Never miss a deadline again

Polling station sign

Thinking about a maths degree?

Chat with other maths applicants

Can you help? Study help unanswered threads

Groups associated with this forum:

View associated groups

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.