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Solve this equation

  1. May 11, 2010 #1
    I'm not sure how to approach this type of equation. Can anyone help me out?

    Solve the equation:

    n/n+3 + 7/n+4 = 1
     
  2. jcsd
  3. May 11, 2010 #2

    Cyosis

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    You want to get rid of the denominators first, then collect terms.
     
  4. May 11, 2010 #3
    I'm still stuck I'm afraid. Getting rid of the denominators is easy is thety are the same but as these are different I need help with the method.
     
  5. May 11, 2010 #4

    Cyosis

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    You can bring them under the same denominator by cross multiplying. Hint: what method would you use to add 1/3+1/4 together?
     
    Last edited: May 11, 2010
  6. May 11, 2010 #5
    I would convert them both to a common demoninator, namely 12, so they would be easier to add: 4/12 + 3/12 = 7/12
     
  7. May 11, 2010 #6

    Cyosis

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    Exactly, now apply the same procedure to the two fractions in your equation.
     
  8. May 11, 2010 #7
    Does that mean that the next stage is to multiply the top and bottom of n/n+3 by 4, (giving 4n/n + 12) and the top and bottom of 7/n+4 by 3 (giving 21/n + 12)? We can then multiply both fractions by n + 12 leaving us with 4n + 21 = 1?
    Am I on the right track?
     
  9. May 11, 2010 #8

    Cyosis

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    No, we're not dealing with 1/3 and 1/4 in your equation. Post #5 was just an example. However you must use the same method. Perhaps it is instructive if you tell me exactly how you converted 1/3+ 1/4 into 7/12 (step by step). Then copy that method for denominators n+3 and n+4 instead of 3 and 4.
     
  10. May 11, 2010 #9
    the LCM of 3 and 4 is 12, so 1/3 and 1/4 can both be expressed as twelfths, namely 4/12 and 3/12.
    n+3 + n+4 = 2n + 7. Is that the common denominator I should be using?
    maybe 7 + n / 2n + 7?
     
  11. May 11, 2010 #10

    Cyosis

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    Where does n+3+n+4 come from? You don't claim that the common denominator for 1/4+1/3 is 3+4 so why would it be for the exercise at hand. The objective is to find a common denominator so you can add the two fractions together. For numbers you're doing it correctly, however n is just a number therefore n+3 is just a number and n+4 is just a number. The rules of mathematics don't suddenly change when you pick another number.

    Perhaps an intermediate step. lets define a=n+3 and b=n+4. Can you add 1/a+1/b together in terms of a and b?
     
  12. May 11, 2010 #11
    Gringo, the least common MULTIPLE of two numbers is a PRODUCT.
    Since "n + 3" and "n + 4" are relatively prime, we must multiply them together to get their LCM. This is analogous to LCM(3,4) = 12 (NOT SEVEN!!!)

    If you multiply the entire equation by the LCM/LCD, there will be no more fractions.
    (But there will be a quadratic equation to solve...)
     
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