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April 21, 2014

April 21, 2014

Posted by **Ren** on Thursday, October 25, 2012 at 1:44am.

- Calculus (Urgent Help) -
**jv**, Thursday, October 25, 2012 at 1:54amI believe that it is asking you to take the derivative of the given function, then set it equal to zero. Remember that when you take the derivative to use the product rule. Then find all points on the unit circle between 0 and 2pi and you should come out with 7 x values. Hope this helps.

- Calculus (Urgent Help) -
**jv**, Thursday, October 25, 2012 at 1:55amI believe that it is asking you to take the derivative of the given function, then set it equal to zero. Remember that when you take the derivative to use the product rule. Then find all points on the unit circle between 0 and 2pi and you should come out with 7 x values. Hope this helps.

- Calculus (Urgent Help) -
**Reiny**, Thursday, October 25, 2012 at 2:15pmfirst of all re-write it as

y = cosx ( 2 ln(cosx) ) using log rules

= 2 cosx ln(cosx)

now use the product rule to find

dy/dx = 2cosx (-sinx/cosx) + (-2sinx)(ln(cosx))

= - 2sinx(1 + ln(cosx) )

= 0 for horizontal tangents

-2sinx = 0 or 1 + ln(cosx) = 0

for -2sinx = 0

sinx = 0

x = 0, π , 2π

for 1 + ln(cox) = 0

ln(cosx) = -1

cosx = e^-1 = 1/e

x = 1.194 or 2π - 1.194

I only found 5 values, don't know how they got the 7 values.

I was going to replace cos^2 x with (cos 2x + 1)/2

but ran into an awful mess trying to solve the derivative for zero

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