Basic functions: Polynomial functions
Solving quadratic inequalities via the quadratic formula and inspection
A quadratic inequality can be solved in three steps:
First we take a value , say . The value of the left-hand side of the inequality is then
Next we choose a value , say . The value of the left-hand side of the inequality is then
Finally we choose a value , say . The value of the left-hand side of the inequality is then
- first solving the corresponding quadratic equation;
- then figuring out in which area(s) the inequality is true;
- finally, combining the intermediate results.
We have the inequality
but first we solve the following equation: , that is . We do this via the quadratic formula:
So
Now we explore where the inequality is true.
First we take a value , say . The value of the left-hand side of the inequality is then
The value of the right-hand side is
So we have found for that .
Next we choose a value , say . The value of the left-hand side of the inequality is then
The value of the right-handside is
So we have found for that .
Finally we choose a value , say . The value of the left-hand side of the inequality is then
The value of the right-hand side is
So we have found for that .
So we can conclude that
when or .
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