Posted by **Samantha** on Tuesday, September 9, 2008 at 8:41pm.

I'm having trouble with inequalities. Basically my problem is about which way the sign goes. Here's an example..

X(X+1)>2

so then it becomes

X^2 + X>2

Correct? Then

X^2 + X - 2>0

then factor so it becomes

(X+2)(X-1)>0

And so I end up with X>-2 and X>1 as my answer, but the book says it should be X<-2. Why did the sign switch? I didn't divide by a negative did I? Can someone help me please?

- Algebra -
**Reiny**, Tuesday, September 9, 2008 at 10:12pm
Here is the logical analysis:

(X+2)(X-1)>0 says that the answer is positive, so either

x+2 > 0 AND x-1 > 0

x > -2 AND x > 1 ----> x > 1

**or**

x+2 < 0 AND x-1 < 0

x < -2 AND x < 1 ----x < -2

so x < -2 OR x > 1

I use the following quick way to solve these kind of problems.

from (X+2)(X-1)>0

the "critical values", or the solution to the corresponding "equation", are

x = -2 and x = 1

put these values on a number line, this will split the number line into 3 regions

1. x < -2

2. x between -2 and 1

3. x > 1

select any value of x in those regions and test it in your factored inequation.

e.g. for

1. I picked x=-5 that works

2. let x = 0 , does NOT work

3. let x = 10 , that also works

so x > 1 OR x < -2

## Answer This Question

## Related Questions

- L'Hopital's rule - Find lim x->1+ of [(1/(x-1))-(1/lnx)]. Here is my work...
- algebra - Graph the system of inequalities x (lesser or equal to sign)2 y (...
- math,drbob222 - I'm sorry the problems i posted had a couple of errors how i ...
- Math - Can someone correct it Evaluate the following expressions numbers 3-4 3...
- math - When you are solving inequalities, when does the sign change and when ...
- Algebra - Hi I am learning about rational expression and so far I am pretty good...
- Math Ms. Sue Please help! - Can you find me a website how to do inequalities on ...
- AP calculus BC - hello i know that deriv(cscx) = -cscxcotx and that deriv(cos...
- math-inequalities - I really don't understand this stuff for example: 9b-3b+34&...
- algebra - Not sure if I have the correct answer dealing with inequalities and ...

More Related Questions