SOLUTION: I need help with my sons Algebra II, it has been many years... Solve the inequality algebraically x 2 -3x-4 < 0 (that is x squared)

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Question 113449: I need help with my sons Algebra II, it has been many years...
Solve the inequality algebraically

x 2 -3x-4 < 0 (that is x squared)

Found 2 solutions by MathLover1, josmiceli:
Answer by MathLover1(20850)   (Show Source): You can put this solution on YOUR website!


We need to solve it as an equation and find roots.


Here is the solution with graph:

Solved by pluggable solver: Quadratic Formula
Let's use the quadratic formula to solve for x:


Starting with the general quadratic





the general solution using the quadratic equation is:







So lets solve ( notice , , and )





Plug in a=1, b=-3, and c=-4




Negate -3 to get 3




Square -3 to get 9 (note: remember when you square -3, you must square the negative as well. This is because .)




Multiply to get




Combine like terms in the radicand (everything under the square root)




Simplify the square root (note: If you need help with simplifying the square root, check out this solver)




Multiply 2 and 1 to get 2


So now the expression breaks down into two parts


or


Lets look at the first part:





Add the terms in the numerator

Divide


So one answer is






Now lets look at the second part:





Subtract the terms in the numerator

Divide


So another answer is




So our solutions are:

or




As you can see, your solutions are: and
For these values will be equal to . That means for all values of from (excluding ) to (excluding ), you will have less than .
or for
As you can see on graph, that part of the parabola lies in III-th and IV-th quadrant (under x-axis)



Answer by josmiceli(19441)   (Show Source): You can put this solution on YOUR website!


OK, I've got two things multiplied together giving me less than zero
which means negative. This is like

Here are the possibilities:
(+)*(+) = +
(-)*(-) = +
(+)*(-) = -
(-)*(+) = -
I don't want the 1st two cases, only the 2nd two.
How do I make (x + 1) negative? By making x less than -1

But if , (x - 4) ends up being negative, too and I get
the case (-)*(-) = +, and I don't want that
So, I'll choose x so that (x - 4) is negative
makes it negative, but I want (x + 1) to stay positive, too
(x + 1) stays positive when
Now I combine these

Notice I only can use the case (-)*(+) = -
Hope you can follow this & hope I got it right, too

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