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

I What does negative ratio means in mathematics?

  1. Apr 9, 2017 #1
    Whats the difference between (4/3) and -(4/3)

    Why ratio in external division is negative?

    I have answer for how it is negative but not why?
    X = (mx2+nx1)/(m+n)

    ⇒ (mx2+nx1) = X(m+n)
    ⇒m(x2-X) = n(X-x1)
    ⇒(m/n) = (X-x1)/(x2-X)

    From the above equation we can conclude that when division is external then X must be greater than x1 and x2, therefore ratio becomes negative
    But whats the purpose of negative ratio.
  2. jcsd
  3. Apr 9, 2017 #2


    User Avatar
    Science Advisor
    Gold Member

    A ratio is negative when the divisor and dividend are of different sign.
  4. Apr 9, 2017 #3
    Distance can never be negative then why external division gives negative ratio.
  5. Apr 9, 2017 #4


    User Avatar
    Science Advisor
    Gold Member

    You can't calculate a positive definite distance without having an absolute value expression ##|x_1 - x_2 |## or a Pythagorean type formula that has squares inside a square root.
  6. Apr 9, 2017 #5


    User Avatar
    2016 Award

    Staff: Mentor

    What do you mean by "external" division? Divisions have a numerator and a denominator. If their signs are different, the result of the division is negative.
    If X>x2 and X>x1, then either m or n has to be negative. So what?
  7. Apr 9, 2017 #6
    In my above example there are 3 points let us name them P(x1,y1), Q(x2,y2), and let a point R(X,Y) lies between P and Q and divides PQ in the ratio PR/QR = m/n, as given in diagram.
    Here R divides PQ internally in the ratio (m/n) when R goes along PQ then it divides PQ externally in the ratio m/n, but when it goes along PQ its cordinate X becomes greater than x1& x2, and y becomes greater than y1 & y2, and by using my above reasoning we get to know that m/n becomes negative, how can the ratio of distance become negative
  8. Apr 9, 2017 #7


    User Avatar
    Science Advisor
    Gold Member

    In your diagram, (m/n) = (X-x1)/(x2-X) is arranged so that it is positive as long as R is between P and Q. The "distances" X-x1 and x2-X are really vectors, not distances, because the direction counts. As soon as R goes outside of the [P,Q] segment, one of the directions changes and the ratio becomes negative.
  9. Apr 9, 2017 #8
    You are using wrong formula, the right one is ##x = {mx_2 - nx_1 \over m -n}##. The rest is explained by fact checker.
  10. Apr 9, 2017 #9
    Hey i know that this is the formua for external division, but i used the above formula to show that how ratio m/n becomes negative when point R goes along PQ and divides PQ externally. By using your formula we can't show that m/n is negative, check it.
  11. Apr 9, 2017 #10
    Sorry to all for this question, i got the idea, i just forget the modulus, (x-x1)/(x2-x) = |m/n|
    Therefore [m/n] = ±[(x-x1)/(x2-x)] = , when its negative it will divide externally otherwise internally
    Last edited: Apr 9, 2017
  12. Apr 10, 2017 #11
    Why do you want to show m/n is negative purposely ?
  13. Apr 10, 2017 #12
    Hey thats not purposely, m/n is always negative in external divison of line, just read the concept
  14. Apr 10, 2017 #13


    User Avatar
    Science Advisor
    Gold Member

    There are better ratios to use that are useful for interpolating between the endpoints and also for extrapolating beyond the endpoints.
  15. Apr 10, 2017 #14
    It is negative because you need to use the correct formula not the wrong one.

    For a ratio (-m/n), the division is ##-mx_2 + nx_1 \over n - m## compare it with ##x_1n - mx_2\over n -m##(I applied the formula for m/n).
    Both are same thing, you are just creating a fuss over nothing.
    Last edited by a moderator: Apr 10, 2017
  16. Apr 11, 2017 #15
    How can you take ratio of two distance as (-m/n) and substitute in the formula, ratio of distance can't be negative.
  17. Apr 11, 2017 #16


    User Avatar
    Science Advisor

    Why not?
  18. Apr 11, 2017 #17
    How can you divide a line in the ratio -(4/3)
  19. Apr 11, 2017 #18
    Yes you can.

    Direction matters. When you write ##m/n## it means you are preserving the direction. To only account for distance you should write ##|m/n|## and then tell explicitly what you need to do with this ratio (divide the segment externally or internally) and then use the correct formula accordingly (See the derivation of those formulae). The benefit in writing ##m/n## is that you don't need to worry about that extra bit of information (nature of division) explicitly.

    Isn't this what @FactChecker said ?
    Last edited: Apr 11, 2017
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Discussions: What does negative ratio means in mathematics?