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Please help: Expressions do not seem equal!

  1. Nov 24, 2007 #1
    In order to see the problem here, you need to refer to following link:

    http://img.photobucket.com/albums/v224/TurbineSpeed/Picture402887.jpg

    I understand how they got the expression in the numerator. What I cannot figure out is how they got the denominator.

    By their math, (x-4)(x+1) + (x+1)(x-4) = (x+1)(x-4)

    This does not seem correct. Give a value to x such as x = 6, and:

    (2)(7) + (7)(2) = 14 + 14 = 28

    Now if we apply this value to the right member, we get:

    (7)(2) = 14.

    14 is NOT equal to 28.

    What am I doing wrong???
     
    Last edited: Nov 24, 2007
  2. jcsd
  3. Nov 24, 2007 #2
    Holocene,
    a/b + c/b = (a+c)/b
    As an example, if you have an eighth of a pizza and have another (one-eighth sized) slice of pizza, you have eaten 2/8=1/4 of the pizza, not 2/16 = 1/8.
     
  4. Nov 24, 2007 #3
    Slider,

    Thank you, but how can (x-4)(x+1) + (x+1)(x-4) be equal to (x+1)(x-4)??????
     
  5. Nov 24, 2007 #4
    WOW! Apparently I had forgotten that when you add fractions you do NOT add the denominators together. Instead, they must simply be equal. Man, I feel stupid for being tripped up over this one.

    Slider, thanks, I can't believe I forgot this simple rule.
     
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