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(x^3-3x^2)-(2x-6)=0 What is x?

Guest Aug 25, 2015

Best Answer 

 #2
avatar+90023 
+10

Here's another approach:

 

(x^3-3x^2)-(2x-6)=0     factor  by grouping

 

x^2(x - 3) - 2(x -3)  = 0

 

(x^2 - 2) (x - 3) = 0        set each factor to 0  ....     and  we find that x = ±√2, 3

 

Here's a graph.......

 

 

CPhill  Aug 25, 2015
 #1
avatar+93677 
+5

$$\\(x^3-3x^2)-(2x-6)=0\\
x^3-3x^2-2x+6=0$$

 

Now use remainder theorem to find a root

$$\\let\\
f(x)=x^3-3x^2-2x+6\\
f(\pm1)=\pm1-3\mp2+6\;\;=\;\;3\pm1\mp2\;\;\rightarrow\;\;2\ne0\;\;or\;\;4\ne 0\\
f(3)=27-27-6+6=0\qquad therefore\;\;x=3 \;\;$is a root$$$

 (this has been edited a little)

 

I used polynomial division to determine that

$$\\(x^3-3x^2-2x+6)\div(x-3)=(x^2-2)\\\\
so\\\\
x^3-3x^2-2x+6=0\\\\
(x-3)(x-\sqrt2)(x+\sqrt2)=0\\\\
x=3\quad or\quad x=\sqrt2\quad or \quad x=-\sqrt2$$

Melody  Aug 25, 2015
 #2
avatar+90023 
+10
Best Answer

Here's another approach:

 

(x^3-3x^2)-(2x-6)=0     factor  by grouping

 

x^2(x - 3) - 2(x -3)  = 0

 

(x^2 - 2) (x - 3) = 0        set each factor to 0  ....     and  we find that x = ±√2, 3

 

Here's a graph.......

 

 

CPhill  Aug 25, 2015
 #3
avatar+93677 
+5

Yes, CPhill's way is better :)

Melody  Aug 25, 2015
 #4
avatar+90023 
0

LOL!!!....Melody always says that she likes to "complicate" things.....actually, both approaches are good .... the "Rational Zeros" approach is always helpful if it's not immediately obvious how to factor a polynomial [if it's actually capable of being factored, that is ]

 

 

CPhill  Aug 25, 2015
 #5
avatar+1312 
+5

Miss Melody is there a difference between these plus minus things?

$$\pm \;\mp$$

I not ever see the one on the right before.

Dragonlance  Aug 25, 2015
 #6
avatar+93677 
+5

 

That is a good question Dragonlance.

I have laid it out a little better now so hopefully it is a little clearer    

 

$$f(\pm1)=\pm1-3\mp2+6\;\;=\;\;3\pm1\mp2\;\;\rightarrow\;\;2\ne0\;\;or\;\;4\ne 0\\$$

 

The signs on the top belong with f(+1)

and the signs on the bottom belong with f(-1)

Melody  Aug 25, 2015

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