Evaluating Chosen Numbers in Quadratic Inequality

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In summary, two real roots of a function represent intervals on either side of the roots, rather than in between.
  • #1
mathdad
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Solve the inequality.

2x^2 + 7x + 5 > 0

Factor LHS.

2x^2 + 2x + 5x + 5 > 0

2x(x + 1) + 5(x + 1) > 0

(2x + 5)(x + 1) > 0

2x + 5 = 0

2x = -5

x = -5/2

x + 1 = 0

x = -1

Plot x = -5/2 and x = -1 on a number line.

<--------(-5/2)----------(-1)----------->

Pick a number from each interval.

Let x = -4 for (-infinity, -5/2).

Let x = -1/2 for (-5/2, -1).

Let x = 0 for (-1, infinity).

Do I avaluate the chosen numbers per interval in the original question or the factored form?
 
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  • #2
Hi RTCNTC,

Plug in your test points into the expression $(2x + 5)(x + 1)$ to determine the signs in the respective intervals. For example, let's consider your test point $x = -4$. Plug in $-4$ for $x$ in the expression $(2x + 5)(x + 1)$ to get $(-7)(-3) = 21$, which has positive sign. Hence, $(2x + 5)(x + 1) > 0$ in the interval $(-\infty, -5/2)$. Now try the other two test points and see what you get.
 
  • #3
Ok. I will continue as you say tomorrow. Going to work now. Math keeps me from drowning in my despair. Thank God for math.
 
  • #4
Pick a number from each interval.

Let x = -4 for (-infinity, -5/2).

Let x = -1/2 for (-5/2, -1).

Let x = 0 for (-1, infinity).

For x = -4, we get true.

For x = -1/2, we get true.

For x = 0, we get true.

We exclude -5/2 and -1.

Solution: (-infinity, -5/2) U (-1, infinity)

Correct?
 
  • #5
Yes, that's correct.
 
  • #6
Very good. What about the other quadratic inequality question? Is that one correct as well?
 
  • #7
In this problem, you have a function representing a parabola that opens upwards. We should then expect, given that it has two real roots, that the intervals on which is is positive will be on either side of the two roots, rather than in between. :D
 
  • #8
Thank you everyone. Two more inequality questions later tonight, and yes, I will show my work.
 

FAQ: Evaluating Chosen Numbers in Quadratic Inequality

1. What is a quadratic inequality?

A quadratic inequality is an inequality that involves a quadratic function, which is a polynomial with a degree of 2. These types of inequalities can be solved by finding the roots of the quadratic equation and analyzing the intervals between the roots.

2. How do you graph a quadratic inequality?

To graph a quadratic inequality, first solve for the roots of the quadratic equation. Then, plot the roots on a number line and determine the intervals between the roots. Next, select a test point from each interval and plug it into the inequality. If the test point satisfies the inequality, then shade the interval in the graph. If the test point does not satisfy the inequality, then leave the interval unshaded.

3. What is the difference between a quadratic equation and a quadratic inequality?

A quadratic equation is an equation that is set equal to 0, while a quadratic inequality is an inequality that is not set equal to 0. In other words, a quadratic equation has an equal sign, while a quadratic inequality has an inequality sign (>, <, ≥, or ≤).

4. Can you solve a quadratic inequality algebraically?

Yes, it is possible to solve a quadratic inequality algebraically by using methods such as factoring, completing the square, or using the quadratic formula. However, graphing the inequality is often a more efficient method for solving quadratic inequalities.

5. What is the solution set of a quadratic inequality?

The solution set of a quadratic inequality is the set of all values of x that satisfy the inequality. This can be represented as an interval or a union of intervals on a number line. The solution set can also be written in set-builder notation, where the variable x is bounded by certain conditions set by the inequality.

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