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Homework Help: Find x very frustrating

  1. Nov 5, 2004 #1
    ok this is a question in some algebra assignment.

    it's drivin' me kinda crazy and nobody will even help in university. this should be easy to solve for some of you i know.

    keep in mind that K1 K2 and K3 and M1 and M2 are all constants that are always positive. K stands for constant of as spring and M is the mass i belive, so they are all positive values.

    here is the question:


    SHOW THAT X1 AND X2 (two possible values of x to solve the equation) ARE BOTH DISTINCT AND NEGATIVE:

    ( X + (K1 +K2)/M1 ) * ( x + (K2 + K3)/m2) - ( -K2/M1) * (-K2/M2) =0


    i simplified it (assumin i made no mistakes) and got :

    x^2 (M1)(M2) + x ( (K1+K3)(M1) + (K1+K2)(M2) ) + (K1)(K2)+ (K1)(K3) +(K2)(K3) =0


    well since all the m's and k's are positive, it's easy for me to prove to him that the x values have to always be negative. i can do this with a statement or whatever. BUT, i'd like to find the values of x (there are two and they are both negative) because it will help me for the later questions. so if anyone knows how to do that, i'd greatly appreciate it.
  2. jcsd
  3. Nov 5, 2004 #2
    does anyone know it..

    or is it even possible

    the teacher assistant i think hinted that it was possible to find the two values of x
  4. Nov 5, 2004 #3
    If you have a quadratic form, why don't you just use the quadratic equation to solve for the roots?

  5. Nov 5, 2004 #4
    is it possible to do a quadratic formula with these values
    like i don't have values for the masses or the k's .
    i will try it
    Last edited: Nov 5, 2004
  6. Nov 5, 2004 #5
    i just put it into the quadratic and it ain't gonna work errrr like the stuff underneath the square root gets way too complicated
  7. Nov 5, 2004 #6
    Nobody ever said it was gonna be pretty.

    And why wouldn't you be able to use the quadratic formula?

  8. Nov 5, 2004 #7
    but underneath the root you have like terms you can't simplify
    and for x i'm supposed to get a number ...
  9. Nov 5, 2004 #8
    How are you supposed to get a number for x when all you have are variables?
  10. Nov 5, 2004 #9
    You can only find the roots in terms of m1,m2,k1,k2,k3. You cannot find numerical roots unless you have numerical values of the above constants.

    Keep in mind that an expression containing m1,m2,k1,k2,k3 will be a number once you substitute in values for m1,m2,k1,k2,k3.

  11. Nov 5, 2004 #10
    ehe ok finally

    i'll just explain to him why x will always be negative though
  12. Nov 5, 2004 #11
    i simplify the equation and get
    [tex] b^2-4ac=(m_{1}m_{2})^2\{[m_{1}(k_{2}+k_{3})-m_{2}(k_{1}+k_{2})]^2+4k^2\}(>0)[/tex]
    So, x1 and x2 are distinct but i can't prove they are negative.
  13. Nov 5, 2004 #12
    x^2 (M1)(M2) + x ( (K1+K3)(M1) + (K1+K2)(M2) ) + (K1)(K2)+ (K1)(K3) +(K2)(K3) =0

    look at this

    you know the first and the last term will be positive right since k and m are always positive

    therefore, the middle term has to be negative, for the equation to =0
    thus, x has to be negative
  14. Nov 5, 2004 #13
    i think you have made a mistake in your simplification.
  15. Nov 5, 2004 #14
    the term with m1m2(k1 +k2)(k2+k3)

    why would u need the mi and m2

    when you multiply you alread have them on the bottom so when you common factor why do u need it up there
  16. Nov 5, 2004 #15
    can someone else factor it plz see if i have it right it shouldn't take u long
  17. Nov 6, 2004 #16
    Leong has proved that the roots are real and distinct (using the discriminant condition). I haven't tried this but I think you should be able to prove that the roots are negative by proving that the product of the roots is negative, i.e the constant term and the coefficient of the second degree term are of opposite signs.
  18. Nov 8, 2004 #17

    ok i'm pretty sure i figured out how to prove that they are negative.

    however, can someone please do similar to what he did above (just for my simplification) to prove that the roots are distinct. I'm sure my simplification is right becuase i confirmed it with numerous classmates.
  19. Nov 8, 2004 #18
    x^2 (M1)(M2) + x ( (K1+K3)(M1) + (K1+K2)(M2) ) + (K1)(K2)+ (K1)(K3) +(K2)(K3) =0

    there is my simplification by the way
  20. Nov 8, 2004 #19
  21. Nov 8, 2004 #20

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