Page 1 of 2 12 LastLast
Results 1 to 15 of 16

Math Help - Simple algebra question - pulling my hair out

  1. #1
    Junior Member
    Joined
    Aug 2008
    Posts
    26

    Simple algebra question - pulling my hair out

    Hi,
    I have a formula i have to use, depending on the data give in the exam i will have to re-arrange it before using it. Problem is i just cant get it work. I my notes i didnt use a lot of detail and i cant remember how i did it in the class, i thought it was so simple at the time, but now i just cant do it! My lecture says its too easy to need worked solutions, so all i have is the answers, not worked solutions.

    f = c ( s - a ) / s ( c - a )

    If the question ask for "f", obviously i can do it, but sometimes it asked for "s".
    When i try to rearrange, i end up getting "s/s" at some point and the formula explodes in my face.

    eg:
    c= 0.98
    a= 0.89
    f= 0.45
    s= ? (answer sheet says 0.928)

    if you are interested the forumla is for calculating the fraction crystallinity in a polymer sample, from densities, its simple linear relationship.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor undefined's Avatar
    Joined
    Mar 2010
    From
    Chicago
    Posts
    2,340
    Awards
    1
    Quote Originally Posted by Siddy View Post
    Hi,
    I have a formula i have to use, depending on the data give in the exam i will have to re-arrange it before using it. Problem is i just cant get it work. I my notes i didnt use a lot of detail and i cant remember how i did it in the class, i thought it was so simple at the time, but now i just cant do it! My lecture says its too easy to need worked solutions, so all i have is the answers, not worked solutions.

    f = c ( s - a ) / s ( c - a )

    If the question ask for "f", obviously i can do it, but sometimes it asked for "s".
    When i try to rearrange, i end up getting "s/s" at some point and the formula explodes in my face.

    eg:
    c= 0.98
    a= 0.89
    f= 0.45
    s= ? (answer sheet says 0.928)

    if you are interested the forumla is for calculating the fraction crystallinity in a polymer sample, from densities, its simple linear relationship.
    Using this page to generate an image from LaTeX:

    Simple algebra question - pulling my hair out-crystallinity.png

    The LaTeX:

    Code:
    \\
    f=\frac{c(s-a)}{s(c-a)}\\
    \\
    fs(c-a)=c(s-a)\\
    \\
    s(fc-fa)=cs-ca\\
    \\
    s(fc-fa-c)=-ca\\
    \\
    s=\frac{-ca}{fc-fa-c}
    Last edited by undefined; June 18th 2010 at 06:03 AM.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Junior Member
    Joined
    Aug 2008
    Posts
    26
    ありがとう ございます!
    Follow Math Help Forum on Facebook and Google+

  4. #4
    MHF Contributor
    Joined
    Dec 2007
    From
    Ottawa, Canada
    Posts
    3,069
    Thanks
    65
    Quote Originally Posted by Siddy View Post
    f = c ( s - a ) / s ( c - a )
    You need another set of brackets! f = c ( s - a ) / (s ( c - a ))

    f = (cs - ca) / (cs - as)
    f(cs - as) = cs - ca
    Can you wrap it up now?
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Junior Member
    Joined
    Aug 2008
    Posts
    26
    The link doesnt work, and i cant find any information about LaTeX maths on google...
    could you repost the link please? i have more algebra...

    1/T = (x /a) + (1-x/b)


    eg.
    1/288 = (x/373) + (1-x/203)

    from trial and error, i get x = 0.65 (and 1-x 0.35). But how do i do it algebraically?
    Follow Math Help Forum on Facebook and Google+

  6. #6
    Super Member
    Joined
    Jun 2009
    Posts
    806
    Thanks
    4
    Quote Originally Posted by Siddy View Post
    The link doesnt work, and i cant find any information about LaTeX maths on google...
    could you repost the link please? i have more algebra...

    1/T = (x /a) + (1-x/b)


    eg.
    1/288 = (x/373) + (1-x/203)

    from trial and error, i get x = 0.65 (and 1-x 0.35). But how do i do it algebraically?
    1/288 = (x/373) + 1/203 - x/203

    x/203 - x/373 = 1/203 - 1/288

    x*( 1/203 - 1/373 ) = 1/203 - 1/288

    Now can you solve for x?
    Follow Math Help Forum on Facebook and Google+

  7. #7
    Junior Member
    Joined
    Aug 2008
    Posts
    26
    Thank you very much, i can do it now!

    so i have to separate the x and then factorise, thanks!
    Follow Math Help Forum on Facebook and Google+

  8. #8
    MHF Contributor undefined's Avatar
    Joined
    Mar 2010
    From
    Chicago
    Posts
    2,340
    Awards
    1
    Sorry about the broken link. Fixed my post above. (The site is here.)
    Follow Math Help Forum on Facebook and Google+

  9. #9
    Junior Member
    Joined
    Aug 2008
    Posts
    26


    I dont understand how to get from step 3 to step 4.
    You subtract "cs" from both sides?
    Shouldnt the left side;
    Follow Math Help Forum on Facebook and Google+

  10. #10
    MHF Contributor undefined's Avatar
    Joined
    Mar 2010
    From
    Chicago
    Posts
    2,340
    Awards
    1
    Quote Originally Posted by Siddy View Post


    I dont understand how to get from step 3 to step 4.
    You subtract "cs" from both sides?
    Shouldnt the left side;
    LaTeX is working on the forum again by the way, using [tex][/tex] tags.

    You could say I skipped a step. Notice that

    s(fc-fa)-cs = s(fc-fa-c)
    Follow Math Help Forum on Facebook and Google+

  11. #11
    Junior Member
    Joined
    Aug 2008
    Posts
    26
    I thought it would be this:

    s(fc - fa) - cs = s(fc - fa - c - 1)<br />
    Follow Math Help Forum on Facebook and Google+

  12. #12
    MHF Contributor undefined's Avatar
    Joined
    Mar 2010
    From
    Chicago
    Posts
    2,340
    Awards
    1
    Quote Originally Posted by Siddy View Post
    I thought it would be this:

    s(fc - fa) - cs = s(fc - fa - c - 1)<br />
    Well no, what you have on the RHS corresponds with s(fc-fa)-cs-s.

    Maybe this will clarify:

    s(fc - fa) - cs = s\bigg((fc - fa)-c\bigg) = s(fc-fa-c)
    Follow Math Help Forum on Facebook and Google+

  13. #13
    Junior Member
    Joined
    Aug 2008
    Posts
    26
    I appreciate your help Undefined.

    I don't understand where the "s" in "cs" go?
    Follow Math Help Forum on Facebook and Google+

  14. #14
    MHF Contributor undefined's Avatar
    Joined
    Mar 2010
    From
    Chicago
    Posts
    2,340
    Awards
    1
    Quote Originally Posted by Siddy View Post
    I appreciate your help Undefined.

    I don't understand where the "s" in "cs" go?
    Maybe it helps to see it all expanded and then factored.

    s(fc - fa) - cs = sfc -sfa -sc = s(fc-fa-c)
    Follow Math Help Forum on Facebook and Google+

  15. #15
    Junior Member
    Joined
    Aug 2008
    Posts
    26
    Thank you so much, i get it now

    I need to practice expanding and factorising...
    Follow Math Help Forum on Facebook and Google+

Page 1 of 2 12 LastLast

Similar Math Help Forum Discussions

  1. Simple Algebra Question
    Posted in the Algebra Forum
    Replies: 3
    Last Post: October 18th 2011, 04:12 PM
  2. Simple question about algebra y^T X y
    Posted in the Advanced Algebra Forum
    Replies: 1
    Last Post: November 17th 2010, 01:22 PM
  3. simple algebra question
    Posted in the Algebra Forum
    Replies: 2
    Last Post: October 6th 2009, 04:35 AM
  4. Simple algebra question.
    Posted in the Algebra Forum
    Replies: 1
    Last Post: September 20th 2009, 06:34 AM
  5. Simple Algebra Question
    Posted in the Algebra Forum
    Replies: 3
    Last Post: March 27th 2009, 01:16 PM

Search Tags


/mathhelpforum @mathhelpforum