Solving inequalities (§4.5)
There is a very general technique for solving inequalities in one variable
that applies to expressions built using
pretty much all of the functions that we consider in this course.
Specifically, it applies to all piecewise-continuous functions.
Exactly what that means is generally explained in a Calculus course,
but I can already tell you what examples we have of these:
any real-valued function made of the following operations
This is a long list,
but there are potential exceptions here:
if you want to solve (−2)x < 1, for example,
then it can be done, but not directly by this method;
the problem is that the base is not positive and the exponent is not constant.
- Addition, subtraction, multiplication, and division;
- Taking opposites, reciprocals, and absolute values;
- Raising to powers whenever the exponent is a constant;
- Extracting roots whenever the index is a constant
(which is usually always the case when people write things with roots);
- Partially-defined or piecewise-defined expressions
whenever the conditions are given by intervals;
- Raising to powers whenever the base is always positive;
- Extracting roots whenever the radicand is always positive;
- Taking logarithms (see Section 5.4 of the textbook);
- Applying any of the trigonometric or inverse trigonometric operations
from Chapters 6 and 7.
Here is the method:
This works because
the only way for the inequality to shift from true to false
is by going through a place where the equation is true or undefined
or by switching from one piece to another in piecewise-defined examples.
In particular, this method applies to
the functions that we study in this course
(linear, power, polynomial, rational, exponential, logarithmic,
linear coordinate transformations thereof, and piecewise versions),
as well as those typically studied in Trigonometry.
- Turn the inequality into an equation and solve it.
(If you get an entire interval of solutions,
then you can just keep the endpoints here.)
- Besides these solutions,
also find when the expressions in the original inequality are undefined.
(Again, if you get an entire interval,
then you can just keep the endpoints.)
- Finally, if you have
a partially-defined or piecewise-defined expression in the inequality,
then find all of the endpoints in the intervals of the pieces' conditions.
- Using the list of numbers found in Steps 1–3,
pick one number between each pair of consecutive numbers in the list,
as well as one number on each side (positive and negative) beyond the list,
as long as the expressions in the inequality are defined there.
- For each of the numbers found in Steps 1–4,
check whether the inequality is true or false there.
- Now you can read off the answer,
letting each number found in Step 4
speak for all of the numbers
in the open interval from which it was chosen.
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This web page was written by Toby Bartels, last edited on 2021 September 24.
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