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 February 12th, 2016, 03:24 PM #1 Newbie   Joined: Feb 2016 From: Oregon Posts: 19 Thanks: 0 Critical Value Help Let f(x) = 8√x − 6x for x > 0. Find the open intervals on which f is increasing/(decreasing). Then determine the x-coordinates of all relative maxima/(minima). Last edited by skipjack; February 12th, 2016 at 11:12 PM.
 February 12th, 2016, 04:00 PM #2 Math Team     Joined: Jul 2011 From: Texas Posts: 2,980 Thanks: 1573 $f(x) = 8\sqrt{x} - 6x$ what did you get as the derivative?
 February 12th, 2016, 08:14 PM #3 Newbie   Joined: Feb 2016 From: Oregon Posts: 19 Thanks: 0 I got f$\,'\!$(x) = 4x - 6 as the derivative. Last edited by skipjack; February 12th, 2016 at 11:04 PM.
 February 12th, 2016, 08:23 PM #4 Math Team   Joined: Dec 2013 From: Colombia Posts: 7,671 Thanks: 2651 Math Focus: Mainly analysis and algebra Not ${4 \over \sqrt x}-6$?
February 13th, 2016, 04:11 AM   #5
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Quote:
 Originally Posted by suhdude I got f$\,'\!$(x) = 4x - 6 as the derivative.
$f(x)=8x^{1/2}-6x$

try taking the derivative of the first term again using the power rule for derivatives ...

$y=kx^n$

$y'=knx^{\color{red}{n-1}}$

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