Skip main navigation

General principles in practice

General principles in practice
11
Hello. In week five, we’re dealing with inequalities. It is quite a delicate topic even for us. In this first step in practice of week five, we are studying the general principles. And more precisely, we are dealing with the sign of a quotient. In exercise one, we first find the domain of inequality, which is the set of x, such that the denominator x minus 2 does not vanish. That is, the set of real numbers except 2. Notice also that for x in the domain, the sign of the quotient x plus 3, times x plus 1, divided by x minus 2 is exactly the sign of the product of these three terms.
63.1
x plus 3 times x plus 1, times x minus 2. And so we use the product rule. So we are going to construct the table with the signs of the three terms– x plus 3, x plus 1, and x minus 2. And then we study the sign of the product. Now the values that vanish x plus 3 is minus 3, x plus 1 vanishes in minus 1 and x minus 2 vanishes in 2. So we’ve got three important values– minus 3, minus 1, and 2. So let us construct the table. And we shall insert here the values minus 3, and minus 1, and 2.
111.3
Here we study the sign of x plus 1, which is 0 at minus 1– positive on the right hand side and negative on the left hand side. And then here we study the sign of x plus 3. And so which vanishes at minus 3. It is positive here and negative there. And then finally the sign of x minus 2, which is 0 in 2– positive on the right hand side and negative elsewhere. Here, the sign of the quotient, x plus 3, times x plus 1, divided by x minus 2. Let us recall that this quotient is not defined at 2, so we put the double line– double vertical line just to remember.
162
And the other signs are exactly the sign– the sign of the quotient is the sign of the product. And so here we have a negative sign, here we have 0, here we have a positive sign, here we get 0 again. Here we have a negative sign and here a positive sign. So in order to have this quotient less or equal than 0, we have to stay on this interval or in this interval, except 2. So the solution is the union of the two sets, of the two intervals minus infinity, up to minus 3. And we take minus 3, union, well, the interval from minus 1 to 2, but we do not take 2. Now, how about exercise 2?
220.8
Well, we have a similar inequality with the difference that instead of x, we’ve got square root of t. So the domain now– D prime– is the set of t– well, such that square root of t is defined. And so we have to impose t greater equal than 0, and also square root of t must be different from 2. So the square root of t, different from 2. That is 0 plus infinity, except t equal 4.
264.8
Now, let us put x equal to square root of t, and we already solved the inequality with x. So we impose that x must belong to minus infinity minus 3, union, minus 1, 2. That is, the square root of t belongs to this union of two intervals. Now, square root of t is positive. So actually, this is equivalent to impose that square root of t belongs to the interval 0, 2.
310.3
And this is equivalent to t belonging to the interval 0, 4. Every element of this interval belongs to the domain of the inequality, and so the solution here is the interval 0, 4.
330.7
So see you in the next step.

The following exercises are solved in this step.

We invite you to try to solve them before watching the video.

In any case, you will find below a PDF file with the solutions.

Exercise 1.

Solve [dfrac{(x+1)(x+3)}{x-2}le 0.]

Exercise 2.

Solve [dfrac{(sqrt t+1)(sqrt t+3)}{sqrt t-2}le 0.]

This article is from the free online

Precalculus: the Mathematics of Numbers, Functions and Equations

Created by
FutureLearn - Learning For Life

Our purpose is to transform access to education.

We offer a diverse selection of courses from leading universities and cultural institutions from around the world. These are delivered one step at a time, and are accessible on mobile, tablet and desktop, so you can fit learning around your life.

We believe learning should be an enjoyable, social experience, so our courses offer the opportunity to discuss what you’re learning with others as you go, helping you make fresh discoveries and form new ideas.
You can unlock new opportunities with unlimited access to hundreds of online short courses for a year by subscribing to our Unlimited package. Build your knowledge with top universities and organisations.

Learn more about how FutureLearn is transforming access to education