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Math Help - Subject of the formula...

  1. #1
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    Subject of the formula...

    Hi there,

    Please could anyone help me out with the following:

    Make E the subject of the formula:

    3 = 7 + 1.52 - E / E

    Am I correct in assuming that you need to multiply both sides by E? However when I do that I get a negative value.

    Thanks for your help!
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  2. #2
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    Re: Subject of the formula...

    It looks like you missed a few parentheses. The way you currently have it written looks like this:

    3 = 7 + 1.52 - \frac{E}{E}

    Looking at that, you can see where you're running into some problems.
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  3. #3
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    Re: Subject of the formula...

    Ah ok, apologies it should read: 3 = 7 + (1.52 - E) / E

    When solving, I understand that you need to collect the variable on one side however I am having difficulty with the denominator "E" and multiplying through.

    3 - 7 = (1.52 - E) / E

    -4 = (1.52 - E) / E

    Then do I multiply both sides by E to get: -4E = E(1.52 - E) ?


    Thanks for the reply!
    Last edited by WoodyGD; September 21st 2011 at 09:18 AM.
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  4. #4
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    Re: Subject of the formula...

    Starting with

    3 = 7 + \frac{1.52-E}{E}

    You can multiply both sides (like you said) by E.

    3E = (7+\frac{1.52 - E}{E}) * E

    Distributing the E on the right side (the left is trivial),

    3E = 1.52-E+7E

    Simplifying,

    3E = 1.52+6E

    Now take it from there, and use your understanding of collecting the variables on one side.
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  5. #5
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    Re: Subject of the formula...

    Right, so you would then get:

    6E - 3E = -1.52

    3E = -1.52

    E = - 1.52 / 3

    E = -0.50666667 ?

    Thanks once again for the help, I really appreciate it!
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  6. #6
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    Re: Subject of the formula...

    Looks good, that's what I got.

    You could have also done it your way (looks like you edited your post while I was responding):

    3 = 7 + \frac{1.52-E}{E}

    Subtract 7 from both sides,

    -4 = \frac{1.52-E}{E}

    Multiply both sides by E,

    -4E = 1.52 - E

    Collect the E terms on the left by adding E to both sides,

    -3E = 1.52

    Divide by -3,

    E = 1.52/(-3)
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  7. #7
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    Re: Subject of the formula...

    Quote Originally Posted by WoodyGD View Post
    Right, so you would then get:

    6E - 3E = -1.52

    3E = -1.52

    E = - 1.52 / 3

    E = -0.50666667 ?

    Thanks once again for the help, I really appreciate it!
    Well, "-0.5066667" is an approximation to the correct answer, of course. In my opinion, there is no point to do addtional work to change from an exact answer to an approximate answer. The best answer is E= -\frac{1.52}{3}.
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  8. #8
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    Re: Subject of the formula...

    Thank you for all of your help! Really appreciate it!
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