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If both sides of an equation are undefined does that mean its true?

  1. May 15, 2013 #1
    I was taking a test and basically for the answer I had limited it to two choices.
    A: x=3 and x does not equal 2
    or
    B. x=3 x=2

    When I plug in three, I got 3/1=12/4 so three is true.
    But when I plugged in two I got 3/0=8/0
    So would this be true too? Or does x not equal 2?
    Wouldnt undefined on both sides be the same as infinity=infinity making it true?
    I already turned my test in and I chose B. Was I right?
     
  2. jcsd
  3. May 15, 2013 #2

    arildno

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    "Wouldnt undefined on both sides be the same as infinity=infinity making it true?"
    Totally wrong. Undefinedness, or infinity for that matter, is not a number, and hence, cannot "Equal" anything, since "equality" is a relation between numbers.

    "I already turned my test in and I chose B. Was I right?"
    No.
     
  4. May 15, 2013 #3

    Mark44

    Staff: Mentor

    Statement A is true if x is equal to 3, and false for all other values of x. If x = 3, the first part is obviously true, as is the second part, since x is not equal to 2.

    Statement B - I can't tell, since there is no connector between the two statements. If the missing connector is "and", then the statement is false, since x can't simultaneously be 2 and 3. If the connector is "or", then statement B is true if the value of x is either 2 or 3. It would be false for any other values of x
    No, 3 is neither true no false - it's a value. Only statements (such as equations or inequalities) can be true or false.
    No
     
  5. May 15, 2013 #4

    mathman

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    It would help if you displayed the equation you are talking about.
     
  6. May 17, 2013 #5
    Even if infinity exists within a number set, it still can't be counted on to equal itself. Infinity is more like a class of numbers then any one specific number. An infinite value might be countable, or uncountable, for example, and those would never equal to each other.
     
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