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Can anyone help me?

 

 

a) x^2-2x-3=0  ;  x^2-2x-3> 0

b) x^2-2IxI-3=0 ; x^2-2IxI-3> 0

c) Ix^2-2x-3I=3

ureeph123  Oct 20, 2014

Best Answer 

 #2
avatar+81027 
+15

a) x^2-2x-3=0  ;  x^2-2x-3> 0

Factoring  x^2-2x-3=0 we have that (x -3)(x +1) =0....and setting each factor to 0, we have that x = 3 and x = -1

For the second part, the polynomial will be greater than 0 either between x= -1 and x = 3 or in the two intervals on either side of this one. If we let x = 0 in the polynomial, note that it makes its value = -3. And this is less than 0. And since 0 falls in the "middle" interval above, the two "outside" intervals will make the inequality true. Thus, x < -1 and x > 3 are the two intervals that "work."

b) x^2-2lxl-3=0 ; x^2-2lxl-3> 0

This one is a little trickier. Let's start by rearranging x^2-2lxl-3=0 ......add 2lxl to both sides....this gives us

x^2 - 3 = 2lxl     divide both sides by 2....so we have

(x^2 - 3 )/ 2 = l x l

We have two equations here....either ...  (x^2 - 3 )/ 2 =  x   or  - (x^2 - 3 )/ 2 = x

Working with the first one, multiply both sides by 2...we have...

x^2 - 3 = 2x    rearranging gives us

x^2 -2x - 3 = 0     and the solution to this is the same as in (a) above. Notice something, however.....x =3 is OK in the original equation, but x = -1 doesn't "work" In effect, we have an extraneous solution.

Now, for the second part, we have - (x^2 - 3 )/ 2 = x   ...multiply both sides by -2...this gives us

x^2 - 3 = -2x        add-2x to both sides

x^2 +2x - 3 = 0      factoring gives  (x +3) (x - 1) = 0   so either x = -3 or x = 1. Note that x = 1 is also extraneous, but x = -3 is OK.

Here's a graph of the equation....https://www.desmos.com/calculator/7rptum7sk3

Notice that it = 0 only at x = -3  and x = 3. This graph will also help us solve the second part of (b). Notice that the function is > 0 on the intervals x< -3 and x > 3.  The odd thing about this graph is that it almost looks like a parabola except for the "kink" at the "vertex." This is due to the absolute value in the function.

c) lx^2-2x-3l=3

To save some time, I'm just going to present the graphs of y = lx^2-2x-3l   and y = 3. Their intersection points will give us a solution.

Here are the graphs......https://www.desmos.com/calculator/9gcr9w4tay

Notice  that the intersection points occur at about x = -1.646, x = 0, x = 2 and x = 3.646. We could have proved this algebraically, but I'm running out of gas, at this point !!!

 

CPhill  Oct 20, 2014
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3+0 Answers

 #1
avatar+26402 
+10

The following should help:

a)

b)

c)

.

Alan  Oct 20, 2014
 #2
avatar+81027 
+15
Best Answer

a) x^2-2x-3=0  ;  x^2-2x-3> 0

Factoring  x^2-2x-3=0 we have that (x -3)(x +1) =0....and setting each factor to 0, we have that x = 3 and x = -1

For the second part, the polynomial will be greater than 0 either between x= -1 and x = 3 or in the two intervals on either side of this one. If we let x = 0 in the polynomial, note that it makes its value = -3. And this is less than 0. And since 0 falls in the "middle" interval above, the two "outside" intervals will make the inequality true. Thus, x < -1 and x > 3 are the two intervals that "work."

b) x^2-2lxl-3=0 ; x^2-2lxl-3> 0

This one is a little trickier. Let's start by rearranging x^2-2lxl-3=0 ......add 2lxl to both sides....this gives us

x^2 - 3 = 2lxl     divide both sides by 2....so we have

(x^2 - 3 )/ 2 = l x l

We have two equations here....either ...  (x^2 - 3 )/ 2 =  x   or  - (x^2 - 3 )/ 2 = x

Working with the first one, multiply both sides by 2...we have...

x^2 - 3 = 2x    rearranging gives us

x^2 -2x - 3 = 0     and the solution to this is the same as in (a) above. Notice something, however.....x =3 is OK in the original equation, but x = -1 doesn't "work" In effect, we have an extraneous solution.

Now, for the second part, we have - (x^2 - 3 )/ 2 = x   ...multiply both sides by -2...this gives us

x^2 - 3 = -2x        add-2x to both sides

x^2 +2x - 3 = 0      factoring gives  (x +3) (x - 1) = 0   so either x = -3 or x = 1. Note that x = 1 is also extraneous, but x = -3 is OK.

Here's a graph of the equation....https://www.desmos.com/calculator/7rptum7sk3

Notice that it = 0 only at x = -3  and x = 3. This graph will also help us solve the second part of (b). Notice that the function is > 0 on the intervals x< -3 and x > 3.  The odd thing about this graph is that it almost looks like a parabola except for the "kink" at the "vertex." This is due to the absolute value in the function.

c) lx^2-2x-3l=3

To save some time, I'm just going to present the graphs of y = lx^2-2x-3l   and y = 3. Their intersection points will give us a solution.

Here are the graphs......https://www.desmos.com/calculator/9gcr9w4tay

Notice  that the intersection points occur at about x = -1.646, x = 0, x = 2 and x = 3.646. We could have proved this algebraically, but I'm running out of gas, at this point !!!

 

CPhill  Oct 20, 2014
 #3
avatar+26402 
+5

Oops!  Just noticed that I assumed zero on the right-hand side for c), but it should have been 3.  Also, no inequality was asked for!

I see Chris has got it right though.

.

Alan  Oct 20, 2014

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