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    Idon't even know where to start. I just went with it but got the wrong answer. Help please.
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    (Original post by Questioness)
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    Idon't even know where to start. I just went with it but got the wrong answer. Help please.
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    Almost, except you have slipped a sign somewhere.

    Remember, when you integrate a function and want to evaluate it between two limits, what do you do?

    Eg [(x^/3) + kx] between 9 and 6.

    You have almost done it but slipped a sign somewhere.
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    (Original post by Questioness)
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    Idon't even know where to start. I just went with it but got the wrong answer. Help please.
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    In the integral between 6 and 9, that should be -k(6) at the end, not +k(6).
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    Why minus? I'm not really aware of any rules about this. My textbook doesn't really explain much.

    (Oh right, I've got the right answer now thanks)
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    (Original post by Questioness)
    Why minus? I'm not really aware of any rules about this. My textbook doesn't really explain much.

    (Oh right, I've got the right answer now thanks)
    Suppose f(x) integrates to F(x), then by the FToC, we have:

    \displaystyle

\begin{equation*} \int_a^b f(x) \, \mathrm{d}x = F(b) - F(a)\end{equation*}
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    (Original post by Zacken)
    Suppose f(x) integrates to F(x), then by the FToC, we have:

    \displaystyle

\begin{equation*} \int_a^b f(x) \, \mathrm{d}x = F(b) - F(a)\end{equation*}
    Is that not what I did?
    F(b) = 243+9k
    F(a)= 72+6k
    Therefore F(b)-F(a) = 243+9k -72+6k
    = 171+15k ?
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    (Original post by Questioness)
    Is that not what I did?
    F(a) = 243+9k
    F(b)= 72+6k
    Therefore F(a)-F(b) = 243+9k -72+6k
    = 171+15k ?
    it's (243+9k)-(72+6k) = 243+9k-72-6k, because you have to negate both terms in the bracket.

    Think of it as -(72+6k) = -1*(72+6k) = -1*72 + -1*6k = -72-6k.
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    (Original post by Questioness)
    Is that not what I did?
    F(a) = 243+9k
    F(b)= 72+6k
    Therefore F(a)-F(b) = 243+9k -72+6k
    = 171+15k ?
    Do you remember your distributive laws? (243 + 9k) - (72 + 6k) = 242 + 9k - 72 - 6k = 171 + 3k, because you're applying the negative sign to the entire bracket (72 + 6k).

    Do you think that -(1 + 1) = ^{?} -1 + 1 = 0? Or do you think that -(1 +1) = -1 -1 = -2?
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    (Original post by HapaxOromenon)
    it's (243+9k)-(72+6k) = 243+9k-72-6k, because you have to negate both terms in the bracket.

    Think of it as -(72+6k) = -1*(72+6k) = -1*72 + -1*6k = -72-6k.
    Ahh okay thanks I get it.
 
 
 
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