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Homework Help: Changing the Subject of a Formula

  1. Oct 2, 2007 #1
    1. The problem statement, all variables and given/known data

    Express the formula for converting Celsius temperatures to Fahrenheit.

    2. Relevant equations

    C= (F+40)(5/9) - 40

    3. The attempt at a solution

    C= (F+40)(5/9) - 40
    C- 40= (F+40 - 40)(5/9) - 40
    (C-40)9/5= F(9/5)(5/9) - 40
    (C - 40)9/5 + 40= F - 40 + 40

    F= (C - 40)9/5 + 40

    Answer: F= (C + 40)9/5 - 40

    Could someone please explain to me why the operation signs in the (C + 40) and - 40 are opposite from the solution I came up with?
  2. jcsd
  3. Oct 2, 2007 #2


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    Staff Emeritus
    Science Advisor

    You made a mistake going from this line to the one below.
    You need to add 40 to both sides.. I don't know where you've got that 40 from inside the brackets. The next line should read

    Now, what do you think you'd do next?
  4. Oct 2, 2007 #3
    OH! I know what I did wrong. I started from within the brackets first and got rid of the +40 next to the F instead of getting rid of the -40 on the outside. Now I see where I went wrong. Thank you.
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