Turn on thread page Beta
    • Thread Starter
    Offline

    0
    ReputationRep:
    Can someone please explain to me, if x=2 + √3 and y=2 - √3, why is (x - y) always irrational? I know that in this case x - y = 2√3 but why does it always happen? I have this proof, (x - y) = (x + y) -2y but dont know what it means :confused:

    Cheers
    Offline

    0
    ReputationRep:
    (Original post by manps)
    Can someone please explain to me, if x=2 + √3 and y=2 - √3, why is (x - y) always irrational? I know that in this case x - y = 2√3 but why does it always happen? I have this proof, (x - y) = (x + y) -2y but dont know what it means :confused:

    Cheers
    Well x+y is rational and 2y is irrational so (x+y) - 2y is irrational. Simpler yet, root 3 is irrational, hence 2root 3 is (derive a simple contradiction if you assume it's rational).
    Offline

    8
    ReputationRep:
    (Original post by manps)
    Can someone please explain to me, if x=2 + √3 and y=2 - √3, why is (x - y) always irrational? I know that in this case x - y = 2√3 but why does it always happen? I have this proof, (x - y) = (x + y) -2y but dont know what it means :confused:

    Cheers
    (x-y) is always irrational because (x-y) is always 2√3.
    √3 is irrational, so its multiple is irrational also.
    Offline

    2
    ReputationRep:
    You can prove √3 is irrational simply. But simply stating that is is irrational is sufficient.
    Offline

    0
    ReputationRep:
    You say "why is x-y always irrational?" ------- you are right, in this case it DOES equal 2root3. HOWEVER...it will not always equal two root 3....and sometimes x-y may be rational...depending on the values of x and y. If x is 8, and y is 5, then x-y=3 which is rational! However, if any irrational numbers are involved and then subtracted, and they don't cancel out (i.e. root 3 - root 3 = 0)then you will be left with an irrational number.

    The "proof" you have suggested has been thought up because someone spotted they needed to get rid of that root 3 for it to become rational. To do this, if x=2+root3 and y=2-root 3, then adding them cancels the root 3s (described above). We now have x+y = 4. BUT we want x-y. To get x-y (from x+y) we now have to subtract 2y...so the answer is 4-2y, but y is 2-root 3...so we have 4-2(2-root3).

    This is 4-4+2root3
    Which is 2 root 3, as before.

    However hard we try to get rid of those root 3's by adding/subtracting, we won't. You CAN get rid of them though... multiply x and y together:

    xy = (2+root3)(2-root3)
    xy = 4 - 2root3 + 2root3 - root9
    xy = 4-3
    xy=1

    A rational number!

    Infact i have two questions to mathematicians/you.

    I have said xy = 4-root 9. Surely root 9 is not only 3, but also -3. Surely then, xy = 4-(-3) and so xy can also equal 7!?

    One last question, how do you get those "root" signs up on your computer please!

    Cheers
    "Countdown" Kirk
    Offline

    8
    ReputationRep:
    (Original post by hatton02)
    I have said xy = 4-root 9. Surely root 9 is not only 3, but also -3. Surely then, xy = 4-(-3) and so xy can also equal 7!?
    It all depends what you mean when you say root(9).
    The (square) root of 9 is both '+3' and '-3', since both (-3)² = 9 and (+3)² = 9.
    But when you write down root(9) or √9, then you are using a function, and the function you are using is defined to give the positive root only.
    When/If you want to write down both roots of 9 then you would write +√9 (or simply √9) or -√9, with +√9 = 3, and -√9 = -3.
    If you see an expression involving surds (roots) then you always take the positive value. That's why you use the ± sign in the quadratric formula, x = -b ± √(b² - 4ac) (all over) 2a, to show that both the positive and negative roots of the discriminant (b² - 4ac) are taken.

    (Original post by hatton02)
    One last question, how do you get those "root" signs up on your computer please!

    Cheers
    "Countdown" Kirk
    Some of the maths symbols are gotten by using the Alt key and the numeric keypad. Others are HTML constructs.
    ± is Alt+241
    √ is &#8730 ; <- but no space between the last digit and the ';'.
    Offline

    8
    ReputationRep:
    Special Characters
    Using the Alt key and the Numeric Keypad

    x² : 253
    x³ : 252
    ± : 241
    ÷ : 246
    3° : 248
    ¤ : 207
    ø : 155
    Ø : 157
    ½ : 171
    ¼ : 172
    « : 174
    » : 175
    µ : 230

    Using HTML constructs
    These constructs take the form of &#dddd; where dddd is a 3- or 4-digit number given as follows.

    Δ : 916
    ≡ : 8801
    η : 951
    ¾ : 190
    ∫ : 8747
    λ : 955
    ≠ : 8800
    ω : 969
    ∂ : 8706
    π : 960
    √ : 8730
    ρ : 961
    ∑ : 8721
    σ : 963
    ε : 949

    You can go to this web-site for more HTML constructs for other symbols.
    http://www.chami.com/tips/internet/050798I.html
    Offline

    0
    ReputationRep:
    test test:

    ≠ and ∑ oh and λ for fun

    Why don't these work on MSN messenger?

    Cheers "Fermat"!
    Kirk
 
 
 
Turn on thread page Beta
Updated: July 10, 2004

University open days

  1. Loughborough University
    General Open Day Undergraduate
    Fri, 21 Sep '18
  2. University of Cambridge
    Churchill College Undergraduate
    Fri, 21 Sep '18
  3. Richmond, The American International University in London
    Undergraduate Open Day Undergraduate
    Fri, 21 Sep '18
Poll
Which accompaniment is best?
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.