Find when

Attempt: https://dl.dropbox.com/u/586465/2012...2016.48.57.jpg

the thing is: I get +3 instead of -3.

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- Sep 14th 2012, 06:52 AMjacob93algebra minus
Find when

Attempt: https://dl.dropbox.com/u/586465/2012...2016.48.57.jpg

the thing is: I get +3 instead of -3. - Sep 14th 2012, 07:12 AMemakarovRe: algebra minus
The derivative is correct. .

- Sep 14th 2012, 08:48 AMDevenoRe: algebra minus
first derivative:

.

second derivative:

.

now if f''(x) = 0, the numerator must be 0, so:

.

thus either 4x = 0, and x = 0, or: 3-x^{2}= 0, in which case:

(your error was in taking -2x^{2}+ x^{2}to be x^{2}, instead of -x^{2}). - Sep 14th 2012, 09:03 AMjacob93Re: algebra minus
the last line, "your error was in taking -2x2 + x2 to be x2, instead of -x2", would've sufficed, but gee… Thank you so much!

- Sep 14th 2012, 09:14 AMDevenoRe: algebra minus
perhaps so, but future readers of this thread may not click on, or view the image link you posted.

also, i'm no computational prodigy, but a bit more like a turtle: slow and sure wins the race. so to keep things straight in my mind, i have to verify. every. single. step (i discovered several computational errors before i finished my post). derivatives involving quotients are particularly nasty, because of the many opportunities for sign errors involved.