Posted by Frank 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(8x)).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  MathMate, Sunday, March 6, 2011 at 7:14am
First we calculate
f'(x)=4*((82*x)/(2*sqrt((8x)*x))+1)
Equate it to zero and solve for x
f(x)=0 =>
((82x)/(2sqrt((8x)x))+1)=0
(82x)/(2sqrt((8x)x))=1
(82x)=2sqrt((8x)x)
Square both sides:
(82x)^2=4(8x)*x)
Solve the quadratic to get
x= 4±2√2.
Verify both roots (because we squared) and reject x=42√2
That leaves us with
x=4+2√2.
Calculate f"(x):
(64sqrt(8xx²))/(x^416x^3+64x^2)
Verify that f"(x) <0 at x=4+√2 and hence a maximum.