# Product Rule finding turning point.

• Sep 25th 2010, 01:39 PM
MK47
Product Rule finding turning point.
Hey, I have a test coming up this week and I not sure how to do these two questions.

If y=3x.cosx , show that the truning point of curve occurs only when x=cos x/sin x
I got as far as dy/dx = 3(x-3sin3x+cos3x)=0 but then i dont know what to do after that. I am not sure if thats right (Nerd)

If y=2x^2.cos3x, show that the turning point of curve occurs at point where x=0 and also whenever x= 2cos3x/3sin3x
I got as fas as dy/dx= 2x(x-3sin3x+2cos3x)=0 then i am not sure what to do next.
Thanks.
• Sep 25th 2010, 01:57 PM
skeeter
Quote:

Originally Posted by MK47
Hey, I have a test coming up this week and I not sure how to do these two questions.

If y=3x.cosx , show that the truning point of curve occurs only when x=cos x/sin x
I got as far as dy/dx = 3(x-3sin3x+cos3x)=0 but then i dont know what to do after that. I am not sure if thats right (Nerd)

If y=2x^2.cos3x, show that the turning point of curve occurs at point where x=0 and also whenever x= 2cos3x/3sin3x
I got as fas as dy/dx= 2x(x-3sin3x+2cos3x)=0 then i am not sure what to do next.
Thanks.

$\displaystyle \frac{d}{dx} (3x \cdot \cos{x}) = -3x\sin{x} + 3\cos{x}$

set the derivative = 0