# find all extremum points

• Nov 2nd 2009, 07:23 PM
cgiulz
find all extremum points
$(a) \frac{x^2}{1 + x^4}$ over $(-\infty,\infty).$

$(b) \frac{\ln(x)}{x}$ over $[1,\infty).$
• Nov 2nd 2009, 07:25 PM
VonNemo19
Quote:

Originally Posted by cgiulz
$(a) \frac{x^2}{1 + x^4}$ over $(-\infty,\infty).$

$(b) \frac{\ln(x)}{x}$ over $[1,\infty).$

What are you having trouble with here? What have you tried? Is it taking the derivative? Choosing test values? etc...?
• Nov 2nd 2009, 07:45 PM
cgiulz
Try them out and you'll see. For $(a)$ the criticals are 0,1 and -1, and as $x \rightarrow \infty, f(x) \rightarrow 0,$ so might this be a minimum? Also, +-1 seem to be a single max? Moreover, the first and second derivative test fail..etc
• Nov 2nd 2009, 07:50 PM
cgiulz
I think I got $(a)$ -- 0 is a max and +-1 are neither.
• Nov 2nd 2009, 07:54 PM
VonNemo19
Quote:

Originally Posted by cgiulz
Try them out and you'll see. For $(a)$ the criticals are 0,1 and -1, and as $x \rightarrow \infty, f(x) \rightarrow 0,$ so might this be a minimum? Also, +-1 seem to be a single max? Moreover, the first and second derivative test fail..etc

a) $f'(x)=\frac{2x(1+x^4)-4x^3(x^2)}{(1+x^4)^2}=0\Rightarrow{x(1-x)(1+x)(1+x^2)}=0\Rightarrow$ critical numbers at $x=1,-1,0$

So, evaluate f prime within these intervals and determine its sign.