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Math Help - Working out +C from integration

  1. #1
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    Working out +C from integration

    Hey I was doing an integration question and I got the answer:

    {y}=6{x}-2{x}^2-{x}^3+c the final marking point on the question was to work out c. The point went through the origin so the end answer (which I had to look up) was {y}=6{x}-2{x}^2-{x}^3. How do I work out c if say the curve {y}=6{x}-2{x}^2-{x}^3+c went through (3,2)?
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  2. #2
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    Quote Originally Posted by Envy View Post
    Hey I was doing an integration question and I got the answer:

    {y}=6{x}-2{x}^2-{x}^3+c the final marking point on the question was to work out c. The point went through the origin so the end answer (which I had to look up) was {y}=6{x}-2{x}^2-{x}^3. How do I work out c if say the curve {y}=6{x}-2{x}^2-{x}^3+c went through (3,2)?
    Sub in x=3 and y=2 and solve for c.

    So {y}=6{x}-2{x}^2-{x}^3+c went through (3,2)

    becomes {2}=6(3)-2(3)^2-(3)^3+c \:, \: c = 2+27+18-18= 29
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  3. #3
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    rearrange?

    so literally rearrange to make c the subject and substitute in the x and y values?
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  4. #4
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    Quote Originally Posted by Envy View Post
    so literally rearrange to make {c} the subject and substitute the {x} and {y}?
    Yeah, that's all there is to it.
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  5. #5
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    Thanks

    Cheers, it seems so obvious now
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  6. #6
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    Quote Originally Posted by Envy View Post
    Cheers, it seems so obvious now
    Yeah, it's even simpler when it's the origin because it usually means that c=0. It only really gets harder to decide when you get an initial condition
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