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4=3 True or False? Check it out and guess where's the mistake!

  1. May 2, 2008 #1
    Let say that a+b=c
    this can be writtem as
    4a-3a+4b-3b=4c-3c
    by arranging
    4a+4b-4c=3a+3b-3c
    taking out the constants
    4*(a+b-c)=3*(a+b-c)
    So 4=3



    The mistake is that .... I'll post it soon. i read this from a notebook of my little brother and i firstly agreed to it but by the time i found out why not! Test your knowledge too.
     
  2. jcsd
  3. May 2, 2008 #2

    CRGreathouse

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    You divide by zero in the last step.
     
    Last edited: May 2, 2008
  4. May 2, 2008 #3

    exk

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    what he said ^
     
  5. May 2, 2008 #4

    rock.freak667

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    You can't divide by a+b-c since a+b=c (i.e. a+b-c=0)....
     
  6. May 4, 2008 #5
    Cool! you got it!
    the trick was to know that dividing by zero is impossible.
     
    Last edited: May 4, 2008
  7. May 4, 2008 #6
    I remember my dad showed it to me when I was in 5th or 6t grade. It stumped me, and I showed it to my Math teacher at school who explained the 11th Commandment to me: "You shall never divide by Zero"
     
  8. May 4, 2008 #7
    This is exactly why you never ever divide by a variable. It is horrendous algebra.


    x^2=4x


    What is x?

    75% of students will divide by x and say x=4.
     
  9. May 4, 2008 #8
    Well they're half-right.
     
  10. May 4, 2008 #9

    rock.freak667

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    Then those 75% should always remember that you can only divide by something only if it is not equal to zero!
     
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