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Math Help - Expanding/Factoring Q.

  1. #1
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    Expanding/Factoring Q.

    Alright. I do not know the answer to this question, and need to know.

    If you were to expand (x-7)(x+7) there would be no x-term. Explain this!

    Please someone help!
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  2. #2
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    Quote Originally Posted by Constance
    Alright. I do not know the answer to this question, and need to know.

    If you were to expand (x-7)(x+7) there would be no x-term. Explain this!

    Please someone help!
    There is an x term! FOIL. First, Outer, Inner Last.

    This gives us x*x+7*x-7*x-49=x^2-49. This is a special expansion called the difference of squares.
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  3. #3
    dud
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    I suppose you mean to expand this, there would be no x in the first order.

    <br />
(x-7)(x+7) \Leftrightarrow (x+7)(x-7) \Leftrightarrow (a+b)(a-b)<br />
    This last one there is a theorem/rule which I don't know what is called in English.
    <br />
(a+b)(a-b) \Leftrightarrow a^2 - b^2<br />
    Which gives.
    <br />
(x-7)(x+7) \Leftrightarrow x^2 - 7^2 \Leftrightarrow x^2 - 49<br />
    There you have it, no x of first degree!
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