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April 25, 2014

April 25, 2014

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

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 10:12pmHere 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

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