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Math Help - 2*2=5

  1. #1
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    2*2=5

    how 2*2 can be equal to 5?
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  2. #2
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    2*2

    We start with:
    -20=-20
    16-36=25-45
    We add 81/4 to sides of equation:
    16-36+81/4=25-45+81/4
    42-2*4*9/2+(9/2)2=52-2*5*9/2+(9/2)2
    (22-9/2)2=(5-9/2)2
    Square root of sides:
    2*2-9/2=5-9/2
    2*2=5
    but why the result is this?
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  3. #3
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    Hello afsaneh
    Quote Originally Posted by afsaneh View Post
    We start with:
    -20=-20
    16-36=25-45
    We add 81/4 to sides of equation:
    16-36+81/4=25-45+81/4
    42-2*4*9/2+(9/2)2=52-2*5*9/2+(9/2)2
    (22-9/2)2=(5-9/2)2
    Square root of sides:
    2*2-9/2=5-9/2
    2*2=5
    but why the result is this?
    It's when you take the square root that you need to be careful. Just because a^2 = b^2, it doesn't necessarily mean that a = b. It could be that a = -b.

    It is true that (2^2-\tfrac92)^2 = (5 - \tfrac92)^2, the line I've coloured
    red. This is because, when we evaluate the expressions in each bracket, the one on the LHS is -\tfrac12 and the one on the RHS is \tfrac12. Squaring each of these gives the same answer: \tfrac14. But that doesn't mean that they are equal, does it?

    So remember: when you take a square root, remember the possible minus sign!

    Grandad
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  4. #4
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    very good
    the point is just that.
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