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Linear equations using addition

  1. Mar 20, 2013 #1
    I'm probably making a silly mistake or Wolfram Alpha is lying to me.

    Question: Find the value of c and d.



    Rearranged, simplified and multiply each equation by 2:


    Now find the common multiple which in my case I will use 12:


    Then add them and find what d is worth:



    Now when I plug this back into the equation, I will use the first one:


    [tex]d=1, c=5[/tex]

    What am I doing wrong? Sorry if this is the long winded way to do it.
  2. jcsd
  3. Mar 20, 2013 #2


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    Your answer is correct, as you can verify by plugging ##d = 1## and ##c = 5## into the two given equations.
  4. Mar 20, 2013 #3
    So Wolfram is lying to me it seems?

    Wolfram says the answer is [tex]c=\frac{35}{16}, d=\frac{23}{8}[/tex]
  5. Mar 20, 2013 #4


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    It seems more likely that you didn't enter the problem correctly into Wolfram Alpha.
  6. Mar 20, 2013 #5


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    I think you told Wolfram the second equation was
    $$3c + \frac d 2 = 8$$
  7. Mar 20, 2013 #6
    Yes, looking back that is what is shows under "Input Result" How would I input the correct format?
  8. Mar 20, 2013 #7


    Staff: Mentor

    If you meant this:
    $$ \frac{3c + d}{2}$$

    you should have written it as (3c + d)/2.

    Also, there was some wasted effort when you multiplied the first equation by 2. You didn't need to do that.
  9. Mar 20, 2013 #8
    Use correct brackets.
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