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Math Help - (a + 3b)(a + 5b) - (a + 4b)^2

  1. #1
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    (a + 3b)(a + 5b) - (a + 4b)^2

    Simplify:

    (a + 3b)(a + 5b) - (a + 4b)^2

    My steps:

    - parenthesis first, then exponent
    a^2 + 5ab + 3ab + 15b^2 - a^2 + 16b^2

    - add/subtract like terms
    31b^2 + 8ab

    The answer key given is -b^2. Where did I go wrong?

    Thanks.
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  2. #2
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    Quote Originally Posted by shenton View Post
    Simplify:
    (a + 3b)(a + 5b) - (a + 4b)^2
    My steps:
    - parenthesis first, then exponent
    a^2 + 5ab + 3ab + 15b^2 - a^2 + 16b^2
    - add/subtract like terms
    31b^2 + 8ab
    The answer key given is -b^2. Where did I go wrong?
    Thanks.
    Hello,

    your 2nd line should read:

    a^2 + 5ab + 3ab + 15b^2 - (a^2 +8ab+ 16b^2)

    a^2 + 5ab + 3ab + 15b^2 - a^2 - 8ab- 16b^2

    EB
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  3. #3
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    Thanks for the help!
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