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Solve For B In Terms Of A

  1. May 24, 2008 #1

    LLS

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    Did I do this one correctly?

    1. The problem statement, all variables and given/known data

    If a = f - 2 and b = 4f + 5, what is an expression for b in terms of a?

    It's multiple choice:

    4a
    4a - 4
    4a + 2
    4a + 7
    4a + 13

    3. The attempt at a solution

    a = f - 2 and b = 4f + 5

    add 2 to each side ----> a = f and b = 4f + 7

    my answer is 4a + 7
     
  2. jcsd
  3. May 24, 2008 #2
    You can add 2 to both sides of one equation, but you cannot just add two to one side of two equations. For example, what would happen if a = f, and b = 3f? When f = 1, we have a = 1, b = 3. Add two to both sides. a = f + 2 and b = 3f + 2. Now, to get the same answer of a = 1, we plug in -1 for f. This time, b = -1 instead of 3.

    In this problem, what is the only thing that connects the two equations, i.e. what is the only thing to appear in both equations? How could you use that fact?
     
  4. May 24, 2008 #3

    LLS

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    I am having trouble posting a response-getting an error message. I can't quote the above post.

    "F" is common to both sides. I'm not sure what to do if that is the correct path.
     
  5. May 24, 2008 #4
    Yes, you're right. What is f equal to? How can you get rid of the f in the second equation?
     
  6. May 24, 2008 #5

    tiny-tim

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    Hi LLS! :smile:

    It's a sort of chain-rule:

    To write b in terms of a, you must write b in terms of f, and write f in terms of a.
    So what is f in terms of a? :smile:
     
  7. May 24, 2008 #6

    LLS

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    I need to divide by f on both sides.

    a = -2 and b = 9

    Would that make f = 0? And then b = 5?

    Oy I'm confused.

    I'll be out for the night at a family dinner in a bit. Please don't be upset if I don't get back to to the problem until tomorrow. I do appreciate the help.
     
  8. May 24, 2008 #7
    No problem. Whenever you do anything to one side of an equation, you need to do it to the other side. In your case, dividing by f, what you end up with is

    [tex]\frac{a}{f} = -2\hspace{0.35in}\text{ and }\hspace{0.35in}\frac{b}{f} = 4 + \frac{5}{f}\hspace{0.35in}\text{ if } f \neq 0[/tex]​

    You cannot just divide one side of an equation by something. Always do it to both sides of the equal sign.

    Read Tim's comment again. Why would finding f in terms of a help if you know b in terms of f?
     
  9. May 24, 2008 #8

    HallsofIvy

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    ??? Why in the world would you "need to divide by f"?

    You know that a = f - 2 . Okay, so solve for f "in terms of a": f= ?

    Now just replace f by that in b= 4f + 5.
     
  10. May 25, 2008 #9

    LLS

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    b = 4(f - 2) + 5

    b = 4f - 8 + 5

    b = 4f - 13

    Thank you
     
  11. May 25, 2008 #10

    tiny-tim

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    Nooo … b = 4f + 5, and you must write f in terms of a.

    So f = … ? :smile:
     
  12. May 25, 2008 #11

    HallsofIvy

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    Since that is not any of the given answers, I'm going to hope that was a typo!
     
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