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Homework Help: Finding the function

  1. Dec 14, 2007 #1
    1. The problem statement, all variables and given/known data
    Find a function f such that f'(x)=x^3 and the line x+y=0 is tangent to the graph of f.

    2. Relevant equations
    Need antiderivative

    3. The attempt at a solution
    I found the antiderivative, aka the function, to be (x^4)/4 + C
    Since the tangent like is x+y=0, --> y=-x, so the slope is -1
    If the slope is -1, that means the derivative, x^3 also equals -1
    Plugging in f(-1) gives 1/4 + C

    Now I don't know what to do with that.
  2. jcsd
  3. Dec 14, 2007 #2
    It is saying that the tangent to f is -1, not f itself.

  4. Dec 14, 2007 #3
    Well wouldn't it still be the same slope anyhow?
  5. Dec 15, 2007 #4


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    Science Advisor

    Where (what x) is the slope of x4/4+ C equal to -1? If x+ y= 0 is tangent to y= x4/4+ C they must have the same y value there.
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