X^3-9x Graphing. Finding xints/yints, cps, ips

May 2010
3
0
I have this problem that seems relatively easy but i seem to be messing up somewhere.

First i find x-ints/y-ints
x^3-9x
x(x+3)(x-3)
x=0 x=3 x=-3
y=0

Now the troubling part
Critical Point: F'(x)=0
3x^2-9=0

I use simple quadratic formula and get x values of 18 and -18 (these values don't seem correct) i can't seem to find out what i am doing wrong, any help on this is greatly appreciated.
 

dwsmith

MHF Hall of Honor
Mar 2010
3,093
582
Florida
I have this problem that seems relatively easy but i seem to be messing up somewhere.

First i find x-ints/y-ints
x^3-9x
x(x+3)(x-3)
x=0 x=3 x=-3
y=0

Now the troubling part
Critical Point: F'(x)=0
3x^2-9=0

I use simple quadratic formula and get x values of 18 and -18 (these values don't seem correct) i can't seem to find out what i am doing wrong, any help on this is greatly appreciated.
\(\displaystyle 3x^2-9=0\rightarrow x^2=3\rightarrow x=\pm\sqrt{3}\)
 
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