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Math Help - cubic help

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    cubic help

    we were given a function of y=(x-a)(x-b)(x-c) and were told to add in our own numbers i chose y=(x-8)(x+5)(x-3). can some one show me the working to get it to a cubic and then the derivative
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    Quote Originally Posted by psychosocial for life View Post
    we were given a function of y=(x-a)(x-b)(x-c) and were told to add in our own numbers i chose y=(x-8)(x+5)(x-3). can some one show me the working to get it to a cubic and then the derivative
    lets say it was (x - 8)(x + 5), could you expand that?

    expand the first two first, and then multiply what you got by the (x - 3). how? first multiply everything by the x and then multiply everything by the -3

    when finding the derivative, all that is needed is the power rule: \frac d{dx} x^n = nx^{n - 1}

    and of course, if there are constants in front of the x's, you just leave them and multiply what you got by them since \frac d{dx}[c f(x)] = c \frac d{dx}f(x). here, your f(x) is x^n


    (why did you choose such big numbers? )
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    thanks for the help
    (why did you choose such big numbers? )
    Dont know but regreting it now
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    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by psychosocial for life View Post
    thanks for the help

    Dont know but regreting it now
    okie dokie.

    post your answer when done, we'll check it for you
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    for some reason it still isnt working out right could you please show me the working out so i can workout what im doing wrong. i just cant work it out
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    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by psychosocial for life View Post
    for some reason it still isnt working out right could you please show me the working out so i can workout what im doing wrong. i just cant work it out
    \underbrace{(x - a)(x - b)}_{\text{do these two first}}(x - c) = [x^2 - (a + b)x + ab](x - c)

    now we take the x in the second set of brackets and multiply everything in the first set, after that, we take the -c in the second set of brackets and multiply everything in the first set. so we get:

    \underbrace{x^3 - (a + b)x^2 + abx}_{\text{everything in the first set times }x}~~~ \underbrace{- cx^2 + c(a + b)x - abc}_{\text{everything in the first set times }-c}

    now group the like terms and simplify

    x^3 - (a + b + c)x^2 + (ab + ac + bc)x - abc
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    thank you again i found out what i did wrong. i got x^3-6x^2-31x-120
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    Quote Originally Posted by psychosocial for life View Post
    thank you again i found out what i did wrong. i got x^3-6x^2-31x-120
    it should be + 120
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  9. #9
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    sorry miss type i got +120
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