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Posted by on Friday, March 4, 2011 at 1:48pm.

Please help with this. I submitted it below and was asked to clarify by Henry (hope this is clearer and thank you). I need the first derivative of f(x)=4(x+ sqrt (x(8-x)).And then the second derivative of this to show by the second derivative test that it is a local maximum. This has been difficult to put into form here.Many thanks.

  • calculus - , Sunday, March 6, 2011 at 7:14am

    First we calculate
    f'(x)=4*((8-2*x)/(2*sqrt((8-x)*x))+1)
    Equate it to zero and solve for x
    f(x)=0 =>
    ((8-2x)/(2sqrt((8-x)x))+1)=0
    (8-2x)/(2sqrt((8-x)x))=-1
    (8-2x)=-2sqrt((8-x)x)
    Square both sides:
    (8-2x)^2=4(8-x)*x)
    Solve the quadratic to get
    x= 4±2√2.
    Verify both roots (because we squared) and reject x=4-2√2
    That leaves us with
    x=4+2√2.

    Calculate f"(x):
    -(64sqrt(8x-x²))/(x^4-16x^3+64x^2)
    Verify that f"(x) <0 at x=4+√2 and hence a maximum.

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