Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A problem

  1. Dec 10, 2004 #1
    there are three numbers a,b,c
    and it is given that

    show that
    a^2+b^2+c^2 <=2
  2. jcsd
  3. Dec 10, 2004 #2


    User Avatar
    Science Advisor
    Homework Helper

    I believe there's an error with your sign:it should be:
    [tex] a^{2}+b^{2}+c^{2} \geq 2 [/tex]

    Use the square formula and the fact that the square of a real number is always greater than "0".
  4. Dec 11, 2004 #3

    matt grime

    User Avatar
    Science Advisor
    Homework Helper

    I don't think so, some how. since there is obviously a case where a=b=c and thus a^2+b^2+c^2=1.
  5. Dec 11, 2004 #4


    User Avatar
    Science Advisor
    Homework Helper

    Then what am I doing wrong??
    [tex](a-b)^2 \geq 0 [/tex] => [tex] a^2+b^2 \geq 2ab [/tex] (1).
    [tex](a-c)^2 \geq 0 [/tex] => [tex] a^2+c^2 \geq 2ac [/tex] (2).
    [tex](b-c)^2 \geq 0 [/tex] => [tex] b^2+c^2 \geq 2bc [/tex] (3).

    Add all 3 relations,devide by 2 and u'll be left with:
    [tex]a^2+b^2+c^2 \geq 1 [/tex] (4).

    Which is different than i had previously obtained last night (i was really tired) but it's not what the problem's asking.
  6. Dec 11, 2004 #5


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    The original problem seems a bit tricky.
    I've been playing with one idea as how to prove it:
    1) Define vectors (a,b,c),(b,c,a).
    Then, by our given equality, written as a dot product:
    If I therefore could prove [tex]\cos\theta\geq\frac{1}{2}[/tex]
    I would have solved it..
    Oops, the original problem should probably be solved using Lagrange multipliers.
    Last edited: Dec 11, 2004
  7. Dec 11, 2004 #6
    Wow, I really don't think calc3 is necessary (also, wouldn't you need stricter conditions on a,b,c other than they're between 0 and 1?). I also, now that I'm thinking about it, am not entirely sure how to solve it. This was my original idea:

    0<=a^2+b^2+c^2 (this is a given like the original I think?)
    0<=a^2+b^2+c^2 +2
    but... *sigh* Maybe someone else knows where one could take it from here? Could we put better restrictions on a,b,c?
  8. Dec 11, 2004 #7


    User Avatar
    Science Advisor
    Homework Helper

    No,your put your mind at work only to come up with nothing.U just proved that the sum of the squares is larger than -2,which is more than obvious,since a sum of (real number) squares is always >=0>-2.
  9. Dec 11, 2004 #8
    Well, can someone show that:

    (a-1)^2 <= bc

    Cause from there it's easy ...
  10. Dec 11, 2004 #9
    I thought I had it, but realized at the end I had a small error. Here are some things I did find that seem like they were leading in the right direction (until I got too tired).

    You can start with a>= 1/2 which gives 2a>=1 and maybe use that somehow (this is easy to show).

    If you can get to a+b+c <= 2 the end result is also easy, this seemed like an easier initial target.
  11. Dec 12, 2004 #10
    Okay, I think I've got it. A kind of weird proof, so let me know if anyone sees a flaw.

    First we order the letters in an arbitrary order:

    0 < c <= b <= a <= 1

    -3a^2 <= 0
    2a^2 - 4a^2 - a^2 <= 0

    since c<=a:
    2a^2 - 4ac - c^2 <= 0
    2a^2 + c^2 <= 4ac + 2c^2

    sinc b<=a:
    a^2 + b^2 + c^2 <= 4ac + 2c^2
    a^2 + b^2 + c^2 <= 2ac + 2ac + 2c^2

    sinc b>=c:
    a^2 + b^2 + c^2 <= 2ac + 2ab + 2bc


    a^2 + b^2 + c^2 <= 2
  12. Dec 12, 2004 #11


    User Avatar
    Science Advisor
    Homework Helper

    There's one small problem with this step:
    [tex] 2a^2-4a^2-a^2 \leq 0 [/tex]
    U assume that [itex] c\leq a [/itex],but my guess is that u cannot full justify that:
    [tex] 2a^2 \leq 4ac+c^2 [/tex]
    ,because,as a,b,c are arbitray in the domain (0,1),while 'c'(which u assumed to be the smallest of the 3) could be very small,close to 0,which would mean that your relation would fail,right??
    Last edited: Dec 12, 2004
  13. Dec 12, 2004 #12
    I had a sign error ... I'll have to look it over again, but I think I can do it in a similar way getting the relations right.
  14. Dec 12, 2004 #13

    matt grime

    User Avatar
    Science Advisor
    Homework Helper

    Nothing (it's simply the AM-GM inequality) however this isn't what you said in the previous post is it, as you point out.
  15. Dec 13, 2004 #14
    Okay, now I think I have it. Once again, start with an arbitrary ordering:


    a-c <= 1
    a(a-c) <= 1
    a^2 - ac <= 1
    a^2 + b^2 +c^2 - b^2 - c^2 -ac <= 1
    a^2 + b^2 +c^2 - ab - bc -ac <= 1
    a^2 + b^2 +c^2 <= 2

    Let me know if anyone sees a problem with this. I think I got all the signs right this time.
  16. Dec 14, 2004 #15
    Isn't anyone going to tell me if they think I'm right or wrong?
  17. Dec 14, 2004 #16

    matt grime

    User Avatar
    Science Advisor
    Homework Helper

    I think you're right, though I'd write it as:

    wlog a=max(a,b,c)

    a^2+b^2+c^2 <= a^2+ ab+ac = a^2 + 1-ac <= 1+a^2 <=2.
  18. Dec 14, 2004 #17
    Very nice, Matt !

  19. Dec 14, 2004 #18
    in your second line you multiplied the left side of the inequality with 'a' but forgot to multiply the right side of the inequality with 'a'.
    another thing if you are going with your first assumption:
    then when you reduct c from everything you get:
    and not a-c<=1.
  20. Dec 14, 2004 #19


    User Avatar
    Science Advisor
    Homework Helper

    You suggest a(a-c)<=a, but since a<=1 also, the weaker inequality a(a-c)<=1 is also true and sufficient for the problem. Same thing for your other complaint, a-c<=1-c implies a-c<=1 since c is positive.
  21. Dec 14, 2004 #20
    if you assume a=1 then it's obviously right.

    i would think to solve this question you simply need to prove that a+b+c<=2
    because then when squaring both sides you get (a+b+c)^2<=4 and substracting 2 from both sides get you with a^2+b^2+c^2<=2, and if he does assume a=1 then the other numbers b,c are fractions which their product should with their sum be equal to one which means that b+c is indeed smaller than one and therefore this a+b+c<=2 is correct and so what that deduced from this.

    btw, this question was asked in another forum (israeli one), and it's fair from the poster who post this question not even to say from where it was taken. :grumpy:
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: A problem
  1. A problem (Replies: 2)

  2. A problem (Replies: 1)

  3. A problem (Replies: 15)

  4. Combinatorics problem (Replies: 2)