- #1
LearningMath
- 16
- 0
Homework Statement
Here are two, unconnected example problems, having to do with quadratic inequalities. I'm confused b/c I understand how to solve them with an = sign, but as inequalities the book shows extra steps where signs get reversed and a total of ~ 4 possible answers come up instead of 2. I don't see how or why the sign reversal arises and the second set of options comes up.
I'll go through them sequentially I'm trying to understand the concept, not the particular answer. Please help me understand why the book switches the sign without explaining why! I understand the particular answers but can't derive a rule of thumb. Please help.
Sorry, using the template doesn't make much sense here, but I'm using it b/c it says to...
Thanks a lot!
Example #1) x2>9
Example #2) 2x2 - 4x - 2 < 4
Homework Equations
n/a
The Attempt at a Solution
Steps the book gives for Example #1 (shown literally)
i) X2 > 9
ii) x2 - 9 > 0
iii) (x+3)(x-3) > 0
iv) x - 3 < 0 and x + 3 > 0
-- OR --
x - 3 > 0 and x + 3 < 0
---So---
x > 3
or
x < -3
My questions: The steps in red are what I don't understand. Why does the book switch the signs and add a second option (the "OR" section)? Why does it not just solve it from step 3 forward, as in:
(x+3) > 0 ; (x-3) > 0
x> -3 ; x > 3
And then cross out x > - 3 b/c it doesn't work, leaving only x > 3?
How do we know to also add (or look for) x < -3?
I'm lost.
Where did this second "or" segment come from and the resulting second answer of x < -3, although I understand how it makes sense plugging it in after the fact.Below is example # 2 written out, but my questions are the same - where do the second choices and sign reversals come from and how/when do we know to consider those?
Thanks and sorry for the long post!
---
Second Example:
2x2 - 4x - 2 < 4
i) 2x2 - 4x - 6 < 0
ii) 2(x2 - 2x -3) < 0
iii) 2(x-3)(x+1) < 0
iv) x - 3 < 0 and x + 1 > 0
v) x < 3 and x > - 1
-1 < x < 3