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Math Help - f(x) = -x^3 evaluating for f(a+h)

  1. #1
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    f(x) = -x^3 evaluating for f(a+h)

    When I evaluate f(x)= -x^3 for f(a+h)

    How is it suppose to look? I'm not sure what to do with the negative in front of the x

    f(a+h) = -[(a+h)(a+h)(a+h)]

    or

    f(a+h) = [(-a-h)(-a-h)(-a-h)]
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  2. #2
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    Quote Originally Posted by superduper1 View Post
    When I evaluate f(x)= -x^3 for f(a+h)

    How is it suppose to look? I'm not sure what to do with the negative in front of the x

    f(a+h) = -[(a+h)(a+h)(a+h)]

    or

    f(a+h) = [(-a-h)(-a-h)(-a-h)]
    \displaymaths f(x)=-x^3

    means:

    \displaymaths f(x)=-(x^3)

    so here you want:

    f(x+h)=-( (x+h)^3)=-(x^3+3x^2h+3xh^2+h^3)=-x^3-3x^2h-3xh^2-h^3


    CB
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  3. #3
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    I'm not sure what I'm doing wrong. The answer I get is incorrect.
    f(x) = -x^3
    evaluate for f(a+h)-f(a) / h

    f-(x^3)= -a^3-3a^3h-3ah^2-h^3
    Then when I evaluate for f(a) I get:
    f(a) = -a^3

    After the -a^3 subtract each other out...I factor out an h from the numerator to cancel out an h from the denominator and get:
    -3a^3 - 3ah-h^2
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  4. #4
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    Quote Originally Posted by superduper1 View Post
    I'm not sure what I'm doing wrong. The answer I get is incorrect.
    f(x) = -x^3
    evaluate for f(a+h)-f(a) / h

    f-(x^3)= -a^3-3a^3h-3ah^2-h^3
    Then when I evaluate for f(a) I get:
    f(a) = -a^3

    After the -a^3 subtract each other out...I factor out an h from the numerator to cancel out an h from the denominator and get:
    -3a^3 - 3ah-h^2
    You should get:

    -3a^2-3ah-h^2

    just check your algebra.

    CB
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  5. #5
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    Quote Originally Posted by superduper1 View Post
    I'm not sure what I'm doing wrong. The answer I get is incorrect.
    f(x) = -x^3
    evaluate for f(a+h)-f(a) / h

    f-(x^3)= -a^3-3a^3h-3ah^2-h^3
    Probably a typo. The second term should be "-3a^2h", not "-3a^3h".

    Then when I evaluate for f(a) I get:
    f(a) = -a^3

    After the -a^3 subtract each other out...I factor out an h from the numerator to cancel out an h from the denominator and get:
    -3a^3 - 3ah-h^2
    you continued your typo- this should be -3a^2- 3ah- h^2
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