Easiest Way to Solve Quadratic Inequalities

Solving quadratic inequalities problem is very easy. Seeing how hard this topic in textbooks breaks my heart. This topic is pretty simple but I just don’t understand why they are making it so complicated. This was my assigned topic for report when I was in high school and I was not able to report it in front of the class because I really don’t know how to deal with it. I asked my teacher about this and he also didn’t know how to discuss and deal with it. To make the story short, he spared me. It is also important to know how to write the intervals in symbol if you are not familiar how to use it.

quad ineq1






Let m and n are the roots of this equation since this is quadratic, the general solution set of these inequalities are the following.

Condition: m>n and a>0

Forms                                                              Solution Set
ax^2+bx+c>0                                (-\infty,n)\cup(m,+\infty)

ax^2+bx+c\geq0                           (-\infty,n]\cup[m,+\infty)

ax^2+bx+c<0                                 (n,m)

ax^2+bx+c\leq0                            [n,m]

Worked Problem 1:

Find the solution set of x^2+3x+2>0.

Take not since a>0. We can proceed to finding the roots of this equation. Don’t mind the inequality symbol. The root of x^2-3x+2=0 is 1 and 2. The solution set is (-\infty,1)\cup(2,+\infty)

Worked Problem 2:
For what values of x is the graph of equation 36-x^2=y is below x-axis?

The values of y that will make the graph below x-axis is negative, thus y<0.
The equation given will become 36-x^2<0. Why? Because 36-x^2 is also y.Take note now that a<0. By multiplying -1 to 36-x^2<0 1, a will become greater than 0. x^2-36>0 but the sense of inequality will also change.

The root of this equation is 6 and –6. Solution set is (-\infty,-6)\cup(6,+\infty)

Worked Problem 3:
Find the solution set of x^2-x-20\leq0.
a>0 and the root of equation is 5 and -4. It follows the fourth form of inequality. The solution set is [-4,5]

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