## Section 2.4 Equations and Inequalities as True/False Statements

¶This section introduces the concepts of algebraic equations and inequalities, and what it means for a number to be a solution to an equation or inequality.

### Subsection 2.4.1 Equations, Inequalities, and Solutions

An equation is two algebraic expressions with an equals sign between them. The two expressions can be relatively simple or more complicated:

A relatively simple equation:

A more complicated equation:

An inequality is quite similar, but the sign between the expressions is one of these: \(\lt\text{,}\) \(\leq\text{,}\) \(\gt\text{,}\) \(\geq\text{,}\) or \(\neq\text{.}\)

A relatively simple inequality:

A more complicated inequality:

A linear equation in one variable can be written in the form \(ax+b=0\text{,}\) where \(a, b\) are real numbers, and \(a\ne0\text{.}\) The variable doesn't have to be \(x\text{.}\) The variable cannot have an exponent other than \(1\) (\(x=x^1\)), and the variable cannot be inside a root symbol (square root, cube root, etc.) or in a denominator.

The following are some linear equations in one variable:

(Note that \(r\) is outside the square root symbol.) We will see in later sections that all equations above can be converted into the form \(ax+b=0\text{.}\)

The following are some non-linear equations:

This chapter focuses on linear equations in *one* variable. We will study other types of equations in later chapters.

The simplest equations and inequalities have numbers and no variables. When this happens, the equation is either *true* or *false*. The following equations and inequalities are *true* statements:

The following equations and inequalities are *false* statements:

When equations and inequalities have variables, we can consider substituting values in for the variables. If replacing a variable with a number makes an equation or inequality *true*, then that number is called a solution to the equation.

###### Example 2.4.2 A Solution

Consider the equation \(y+2=3\text{,}\) which has only one variable, \(y\text{.}\) If we substitute in \(1\) for \(y\) and then simplify:

we get a true equation. So we say that \(1\) is a solution to \(y+2=3\text{.}\) Notice that we used a question mark at first because we are unsure if the equation is true or false until the end.

If replacing a variable with a value makes a false equation or inequality, that number is not a solution.

###### Example 2.4.3 Not a Solution

Consider the inequality \(x+4\gt 5\text{,}\) which has only one variable, \(x\text{.}\) If we substitute in \(0\) for \(x\) and then simplify:

we get a false equation. So we say that \(0\) is *not* a solution to \(x+4\gt 5\text{.}\)

### Subsection 2.4.2 Checking Possible Solutions

Given an equation or an inequality (with one variable), checking if some particular number is a solution is just a matter of replacing the value of the variable with the specified number and determining if the resulting equation/inequality is true or false. This may involve some amount of arithmetic simplification.

###### Example 2.4.4

Is \(8\) a solution to \(x^2-5x=\sqrt{2x}+20\text{?}\)

To find out, substitute in \(8\) for \(x\) and see what happens.

So yes, \(8\) is a solution to \(x^2-5x=\sqrt{2x}+20\text{.}\)

###### Example 2.4.5

Is \(-5\) a solution to \(\sqrt{169-y^2}=y^2-2y\text{?}\)

To find out, substitute in \(-5\) for \(y\) and see what happens.

So no, \(-5\) is not a solution to \(\sqrt{169-y^2}=y^2-2y\text{.}\)

But is \(-5\) a solution to the *inequality* \(\sqrt{169-y^2}\leq y^2-2y\text{?}\) Yes, because substituting \(-5\) in for \(y\) would give you

which is true.

###### Checkpoint 2.4.6

###### Checkpoint 2.4.7

###### Checkpoint 2.4.8

###### Checkpoint 2.4.9

###### Checkpoint 2.4.10

###### Example 2.4.11 Cylinder Volume

A cylinder's volume is related to its radius and its height by:

where \(V\) is the volume, \(r\) is the base's radius, and \(h\) is the height. If we know the volume is 96\(\pi\) cm^{3} and the radius is 4 cm, then we have:

Is 4 cm the height of the cylinder? In other words, is \(4\) a solution to \(96\pi=16\pi h\text{?}\) We will substitute \(h\) in the equation with \(4\) to check:

Since \(96\pi=64\pi\) is false, \(h=4\) does *not* satisfy the equation \(96\pi=16\pi h\text{.}\)

Next, we will try \(h=6\text{:}\)

When \(h=6\text{,}\) the equation \(96\pi=16\pi h\) is true. This tells us that \(6\) *is* a solution to \(96\pi=16\pi h\text{.}\)

###### Remark 2.4.13

Note that we did not approximate \(\pi\) with \(3.14\) or any other approximation. We often leave \(\pi\) as \(\pi\) throughout our calculations. If we need to round, we do so as a final step.

###### Example 2.4.14

Jaylen has budgeted a maximum of \(\$300\) for an appliance repair. The total cost of the repair can be modeled by \(89+110(h-0.25)\text{,}\) where \(\$89\) is the initial cost and \(\$110\) is the hourly labor charge after the first quarter hour. Is \(2\) hours a solution for \(h\) in the inequality \(89+110(h-0.25)\le 300\text{?}\)

To determine if \(h=2\) satisfies the inequality, we will replace \(h\) with \(2\) and check if the statement is true:

Thus, \(2\) hours is a solution for \(h\) in the inequality \(89+110(h-0.25)\le 300\text{.}\) In context, this means that Jaylen would stay within their \(\$300\) budget if \(2\) hours of labor were performed.

### Subsection 2.4.3 Exercises

###### Review and Warmup

###### 1

Evaluate \({-9-x}\) for \(x = -4\text{.}\)

###### 2

Evaluate \({5-x}\) for \(x = -2\text{.}\)

###### 3

Evaluate \({-2x+7}\) for \(x = 1\text{.}\)

###### 4

Evaluate \({-10x-5}\) for \(x = 3\text{.}\)

###### 5

Evaluate \({-4\!\left(r+2\right)}\) for \(r = 4\text{.}\)

###### 6

Evaluate \({-10\!\left(t+9\right)}\) for \(t = -3\text{.}\)

###### 7

Evaluate the expression \(\displaystyle \frac{1}{3} \big( x + 4 \big)^2 - 2\) when \(x = -7\text{.}\)

###### 8

Evaluate the expression \(\displaystyle \frac{1}{4} \big( x + 1 \big)^2 - 7\) when \(x = -5\text{.}\)

###### 9

Evaluate the expression \(-16t^{2}+64t+128\) when \(t=-3\text{.}\)

###### 10

Evaluate the expression \(-16t^{2}+64t+128\) when \(t=-5\text{.}\)

###### Identifying Linear Equations and Inequalities

###### 11

Are the equations below linear equations in one variable?

\(\sqrt{9-5.5r}=7\)

is

is not

\(2-3r^{2}=18\)

is

is not

\(x-6q^{2}=27\)

is

is not

\(2\pi r=2\pi\)

is

is not

\(8.59y=-9\)

is

is not

\(12-3z=4\)

is

is not

###### 12

Are the equations below linear equations in one variable?

\(-5-4y^{2}=-21\)

is

is not

\(\sqrt{1-4.7x}=2\)

is

is not

\(9q-y^{2}=1\)

is

is not

\(2\pi r=12\pi\)

is

is not

\(-r-11=3\)

is

is not

\(6.6y=6\)

is

is not

###### 13

Are the equations below linear equations in one variable?

\(4.61q=14\)

is

is not

\(5qVx=-42\)

is

is not

\(V^{2}+y^{2}=-68\)

is

is not

\(\pi r^{2}=48\pi\)

is

is not

\(7-2r=17\)

is

is not

\(r\sqrt{12}=-84\)

is

is not

###### 14

Are the equations below linear equations in one variable?

\(q\sqrt{24}=63\)

is

is not

\(\pi r^{2}=35\pi\)

is

is not

\(-2.96x=-22\)

is

is not

\(-8prz=52\)

is

is not

\(15x-4=28\)

is

is not

\(q^{2}+y^{2}=4\)

is

is not

###### 15

Are the inequalities below linear inequalities in one variable?

\(4p^{2}-y\gt-51\)

is

is not

\(-1\gt1-5p\)

is

is not

\(-3z^{2}-7y^{2}\lt1\)

is

is not

###### 16

Are the inequalities below linear inequalities in one variable?

\(3x^{2}+6z^{2}\gt1\)

is

is not

\(9V^{2}+6p\gt51\)

is

is not

\(4\geq-5-3y\)

is

is not

###### 17

Are the inequalities below linear inequalities in one variable?

\(-5.8x\gt96\)

is

is not

\(-1\gt-4878r-2301p\)

is

is not

\(\sqrt{9p}-9\leq-6\)

is

is not

###### 18

Are the inequalities below linear inequalities in one variable?

\(-6y\lt-43\)

is

is not

\(\sqrt{9r}+6\lt-6\)

is

is not

\(198\leq4182y-9693x\)

is

is not

###### Checking a Solution for an Equation

###### 19

Is \(-3\) a solution for \(x\) in the equation \({x+7} = {3}\text{?}\)

Yes

No

###### 20

Is \(2\) a solution for \(x\) in the equation \({x+9} = {7}\text{?}\)

Yes

No

###### 21

Is \(-9\) a solution for \(r\) in the equation \({-3-r} = {5}\text{?}\)

Yes

No

###### 22

Is \(6\) a solution for \(r\) in the equation \({5-r} = {-1}\text{?}\)

Yes

No

###### 23

Is \(-1\) a solution for \(r\) in the equation \({-7r-8} = {-1}\text{?}\)

Yes

No

###### 24

Is \(-9\) a solution for \(t\) in the equation \({2t+8} = {-10}\text{?}\)

Yes

No

###### 25

Is \(5\) a solution for \(t\) in the equation \({-10t+2} = {-9t-3}\text{?}\)

Yes

No

###### 26

Is \(2\) a solution for \(x\) in the equation \({-2x-4} = {-9x-18}\text{?}\)

Yes

No

###### 27

Is \(-9\) a solution for \(x\) in the equation \({9\!\left(x-11\right)} = {20x}\text{?}\)

Yes

No

###### 28

Is \(3\) a solution for \(y\) in the equation \({2\!\left(y+14\right)} = {9y}\text{?}\)

Yes

No

###### 29

Is \(-3\) a solution for \(y\) in the equation \({-3\!\left(y-13\right)} = {16\!\left(y+6\right)}\text{?}\)

Yes

No

###### 30

Is \(-10\) a solution for \(r\) in the equation \({19\!\left(r+1\right)} = {9\!\left(r-9\right)}\text{?}\)

Yes

No

###### 31

Is \({{\frac{7}{10}}}\) a solution for \(x\) in the equation \({10x-1} = -8\text{?}\)

Yes

No

###### 32

Is \({{\frac{14}{5}}}\) a solution for \(x\) in the equation \({5x-4} = 9\text{?}\)

Yes

No

###### 33

Is \({4}\) a solution for \(t\) in the equation \({-{\frac{10}{3}}t - {\frac{3}{10}}} = {-{\frac{17}{15}}}\text{?}\)

Yes

No

###### 34

Is \({-{\frac{9}{2}}}\) a solution for \(t\) in the equation \({{\frac{2}{5}}t - {\frac{9}{10}}} = {-{\frac{27}{10}}}\text{?}\)

Yes

No

###### Checking a Solution for an Inequality

###### 35

Decide whether each value is a solution to the given inequality.

\(-2 x +9 > 7\)

\(x=-5\)

is

is not

\(x=6\)

is

is not

\(x=0\)

is

is not

\(x=1\)

is

is not

###### 36

Decide whether each value is a solution to the given inequality.

\(2 x - 5 > 1\)

\(x=3\)

is

is not

\(x=0\)

is

is not

\(x=13\)

is

is not

\(x=2\)

is

is not

###### 37

Decide whether each value is a solution to the given inequality.

\(3 x - 8 \ge -5\)

\(x=0\)

is

is not

\(x=8\)

is

is not

\(x=1\)

is

is not

\(x=-5\)

is

is not

###### 38

Decide whether each value is a solution to the given inequality.

\(-3 x +19 \ge 10\)

\(x=2\)

is

is not

\(x=0\)

is

is not

\(x=3\)

is

is not

\(x=13\)

is

is not

###### 39

Decide whether each value is a solution to the given inequality.

\(4 x - 16 \le 4\)

\(x=0\)

is

is not

\(x=10\)

is

is not

\(x=5\)

is

is not

\(x=3\)

is

is not

###### 40

Decide whether each value is a solution to the given inequality.

\(4 x - 10 \le -2\)

\(x=1\)

is

is not

\(x=2\)

is

is not

\(x=12\)

is

is not

\(x=0\)

is

is not

###### Checking Solutions for Application Problems

###### 41

A triangle’s area is \(99\) square meters. Its height is \(11\) meters. Suppose we wanted to find how long is the triangle’s base. A triangle’s area formula is

where \(A\) stands for area, \(b\) for base and \(h\) for height. If we let \(b\) be the triangle’s base, in meters, we can solve this problem using the equation:

Check whether \(36\) is a solution for \(b\) of this equation.

Yes

No

###### 42

A triangle’s area is \(95\) square meters. Its height is \(10\) meters. Suppose we wanted to find how long is the triangle’s base. A triangle’s area formula is

where \(A\) stands for area, \(b\) for base and \(h\) for height. If we let \(b\) be the triangle’s base, in meters, we can solve this problem using the equation:

Check whether \(38\) is a solution for \(b\) of this equation.

Yes

No

###### 43

When a plant was purchased, it was \(2\) inches tall. It grows \(1\) inches per day. How many days later will the plant be \(17\) inches tall?

Assume the plant will be \(17\) inches tall \(d\) days later. We can solve this problem using the equation:

Check whether \(17\) is a solution for \(d\) of this equation.

Yes

No

###### 44

When a plant was purchased, it was \(1.2\) inches tall. It grows \(0.2\) inches per day. How many days later will the plant be \(5.2\) inches tall?

Assume the plant will be \(5.2\) inches tall \(d\) days later. We can solve this problem using the equation:

Check whether \(21\) is a solution for \(d\) of this equation.

Yes

No

###### 45

A water tank has \(163\) gallons of water in it, and it is being drained at the rate of \(7\) gallons per minute. After how many minutes will there be \(37\) gallons of water left?

Assume the tank will have \(37\) gallons of water after \(m\) minutes. We can solve this problem using the equation:

Check whether \(19\) is a solution for \(m\) of this equation.

Yes

No

###### 46

A water tank has \(171\) gallons of water in it, and it is being drained at the rate of \(9\) gallons per minute. After how many minutes will there be \(45\) gallons of water left?

Assume the tank will have \(45\) gallons of water after \(m\) minutes. We can solve this problem using the equation:

Check whether \(17\) is a solution for \(m\) of this equation.

Yes

No

###### 47

A cylinder’s volume is \(126\pi\) cubic centimeters. Its height is \(14\) centimeters. Suppose we wanted to find how long is the cylinder’s radius. A cylinder’s volume formula is

where \(V\) stands for volume, \(r\) for radius and \(h\) for height. Let \(r\) represent the cylinder’s radius, in centimeters. We can solve this problem using the equation:

Check whether \(3\) is a solution for \(r\) of this equation.

Yes

No

###### 48

A cylinder’s volume is \(960\pi\) cubic centimeters. Its height is \(15\) centimeters. Suppose we wanted to find how long is the cylinder’s radius. A cylinder’s volume formula is

where \(V\) stands for volume, \(r\) for radius and \(h\) for height. Let \(r\) represent the cylinder’s radius, in centimeters. We can solve this problem using the equation:

Check whether \(64\) is a solution for \(r\) of this equation.

Yes

No

###### 49

A country’s national debt was \(140\) million dollars in 2010. The debt increased at \(50\) million dollars per year. If this trend continues, when will the country’s national debt increase to \(640\) million dollars?

Assume the country’s national debt will become \(640\) million dollars \(y\) years after 2010. We can solve this problem using the equation:

Check whether \(10\) is a solution for \(y\) of this equation.

Yes

No

###### 50

A country’s national debt was \(100\) million dollars in 2010. The debt increased at \(60\) million dollars per year. If this trend continues, when will the country’s national debt increase to \(1180\) million dollars?

Assume the country’s national debt will become \(1180\) million dollars \(y\) years after 2010. We can solve this problem using the equation:

Check whether \(20\) is a solution for \(y\) of this equation.

Yes

No

###### 51

A school district has a reserve fund worth \(41.1\) million dollars. It plans to spend \(2.9\) million dollars per year. After how many years, will there be \(15\) million dollars left?

Assume there will be \(15\) million dollars left after \(y\) years. We can solve this problem using the equation:

Check whether \(11\) is a solution for \(y\) of this equation.

Yes

No

###### 52

A school district has a reserve fund worth \(32\) million dollars. It plans to spend \(3\) million dollars per year. After how many years, will there be \(11\) million dollars left?

Assume there will be \(11\) million dollars left after \(y\) years. We can solve this problem using the equation:

Check whether \(8\) is a solution for \(y\) of this equation.

Yes

No

###### 53

A rectangular frame’s perimeter is \(6\) feet. If its length is \(2\) feet, suppose we want to find how long is its width. A rectangle’s perimeter formula is

where \(P\) stands for perimeter, \(l\) for length and \(w\) for width. We can solve this problem using the equation:

Check whether \(4\) is a solution for \(w\) of this equation.

Yes

No

###### 54

A rectangular frame’s perimeter is \(7\) feet. If its length is \(2.1\) feet, suppose we want to find how long is its width. A rectangle’s perimeter formula is

where \(P\) stands for perimeter, \(l\) for length and \(w\) for width. We can solve this problem using the equation:

Check whether \(1.4\) is a solution for \(w\) of this equation.

Yes

No