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4.07 Solving rational equations

Lesson

Concept summary

A rational equation is any equation containing at least one rational expression.

When working algebraically to find solutions we can get viable, non-viable, and/or extraneous solutions.

Viable solution

A valid solution that makes sense within the context of the question or problem

Non-viable solution

An algebraically valid solution that does not make sense within the context of the question or problem

Extraneous solution

A solution to an equation that emerges from the algebraic process of solving the problem, but does not satisfy the original equation

For rational equations, extraneous solutions typically occur at values which make both the numerator and denominator of a rational expression equal to 0. That is, for f\left(x\right)=\dfrac{P\left(x\right)}{Q\left(x\right)}, if we have that P\left(a\right)=0 and Q\left(a\right)=0 then x=a is extraneous.

When solving rational equations it is important to keep in mind both the algebraic restrictions on the variables as well as any context involved in the question.

Worked examples

Example 1

For each of the following equations, determine whether x = 3 is a viable solution, an extraneous solution, or not a solution at all:

a

\frac{x - 3}{x + 3} = 0

Solution

By substituting x = 3 into the expression, we can see that\frac{3 - 3}{3 + 3} = \frac{0}{6} = 0

x = 3 is a viable solution to this equation.

b

\frac{x + 5}{x - 3} = 0

Solution

By substituting x = 3 into the expression, we can see that\frac{3 + 5}{3 - 3} = \frac{8}{0} \neq 0

x = 3 is not a solution to this equation.

c

\frac{\left(x - 3\right)^2}{x - 3} = 0

Solution

We can see that x = 3 is a zero of both the numerator and denominator of the rational expression. So while it will be an algebraic solution to the equation (if we multiply both sides by x - 3, for instance), it is also a value which makes the equation undefined.

So x = 3 is an extraneous solution to this equation.

Example 2

Solve the following rational equation: \frac{x^2 + 5x - 24}{x^2 - 9x + 18} = 0

Approach

In order for a rational expression to be equal to 0, its numerator must be equal to 0. So we will want to factor the numerator to solve this equation.

We also want to factor the denominator, however, to check whether each solution is viable or extraneous.

Solution

The expression in the numerator is x^2 + 5x - 24, which can be factored to \left(x + 8\right)\left(x - 3\right).

The expression in the denominator is x^2 - 9x + 18, which can be factored to \left(x - 6\right)\left(x - 3\right).

So we can rewrite the equation as\frac{\left(x + 8\right)\left(x - 3\right)}{\left(x - 6\right)\left(x - 3\right)} = 0

From the denominator, we can see that x cannot be equal to 3 or 6.

From the numerator, we can see that the possible solutions are x = -8 and x = 3.

Putting this together, we have that x = -8 is a viable solution, while x = 3 is an extraneous solution.

Reflection

If we were to solve the equation by first multiplying both sides by x^2 - 9x + 18, and then factoring the remaining quadratic, we would still get the two possible solutions of x = -8 and x = 3.

In order to determine whether they were valid or extraneous, we could substitute into the initial expression \dfrac{x^2 + 5x - 24}{x^2 - 9x + 18}:

For x = -8:\quad\dfrac{\left(-8\right)^2 + 5\left(-8\right) - 24}{\left(-8\right)^2 - 9\left(-8\right) + 18} = \dfrac{64 - 40 - 24}{64 + 72 + 18} = \dfrac{0}{154} = 0

For x = 3:\quad\dfrac{3^2 + 5\left(3\right) - 24}{3^2 - 9\left(3\right) + 18} = \dfrac{9 + 15 - 24}{9 - 27 + 18} = \dfrac{0}{0}

So only the solution of x = -8 is valid. The solution of x = 3 produces an undefined expression and is therefore extraneous.

Example 3

Solve the following rational equation:\frac{1}{x + 6} = \frac{2}{2x - 5}

Approach

Since there are two rational expressions in this equation, we will start by rearranging the equation to have both rational expressions on the same side, equal to zero, and then combine them by making a common denominator.

Solution

\displaystyle \frac{1}{x + 6}\displaystyle =\displaystyle \frac{2}{2x - 5}State the equation
\displaystyle \frac{1}{x + 6} - \frac{2}{2x - 5}\displaystyle =\displaystyle 0Subtract \dfrac{2}{2x - 5} from both sides
\displaystyle \frac{2x - 5}{\left(x + 6\right)\left(2x - 5\right)} - \frac{2\left(x + 6\right)}{\left(x + 6\right)\left(2x - 5\right)}\displaystyle =\displaystyle 0Rewrite each rational expression to have a common denominator
\displaystyle \frac{2x - 5 - 2\left(x + 6\right)}{\left(x + 6\right)\left(2x - 5\right)}\displaystyle =\displaystyle 0Subtract the numerators
\displaystyle \frac{-17}{\left(x + 6\right)\left(2x - 5\right)}\displaystyle =\displaystyle 0Simplify the numerator

At this point, we can see that the equation has no solutions (viable or otherwise), since there is no value of x that can make the numerator of the rational expression be 0.

Reflection

It is also possible to draw this conclusion by rewriting the intial equation\frac{1}{x + 6} = \frac{2}{2x - 5}as\frac{2}{2x + 12} = \frac{2}{2x - 5}At this point the numerators are equal, while the functions in the denominators are linear expressions with the same slope (i.e. parallel lines). So the denominators will never be equal for any value of x, and therefore the equation has no solutions.

Example 4

A farmer has two rectangular chicken coops which share a common border along their longer side.

One coop has an area of 60 square feet and a width of x feet. The other has an area of 96 square feet and is 3 feet longer than the other coop. Determine the dimensions of each coop, showing your work.

Approach

The information that we know about each chicken coop is:

    • Area = 60\text{ ft}^2
    • Width = x\text{ ft}
    • Area =96\text{ ft}^2
    • Width = \left(x + 3\right)\text{ ft}

and we know that they have the same length. We can use this information to draw a sketch:

A diagram showing two connected rectangles sharing a common side of the same length. The vertical length is labelled. The area of the first rectangle is 60 square feet and the other is 96 square feet. The width of the smaller rectangle is x while the bigger is x plus 3.

So we can form an equation by getting an expression for the length of each coop in terms of x and setting them equal to each other, then solve that equation for x. We can then use the value of x to find the widths and length of the coops.

Solution

For a rectangle, we know that \text{Area} = \text{Length} \cdot \text{Width}, which we can rearrange to get \text{Length} = \text{Area} \div \text{Width}.

So the length of the first coop will be \dfrac{60}{x}, while the length of the second coop will be \dfrac{96}{x + 3}.

We can now set these lengths to be equal and solve for x as follows:

\displaystyle \frac{60}{x}\displaystyle =\displaystyle \frac{96}{x + 3}Equate lengths
\displaystyle \frac{60}{x} - \frac{96}{x + 3}\displaystyle =\displaystyle 0Subtract \dfrac{96}{x + 3} from both sides
\displaystyle \frac{60\left(x + 3\right)}{x\left(x + 3\right)} - \frac{96x}{x\left(x + 3\right)}\displaystyle =\displaystyle 0Rewrite each rational expression to have a common denominator
\displaystyle \frac{60\left(x + 3\right) - 96x}{x\left(x + 3\right)}\displaystyle =\displaystyle 0Subtract the numerators
\displaystyle \frac{180 - 36x}{x\left(x + 3\right)}\displaystyle =\displaystyle 0Simplify the numerator
\displaystyle 180 - 36x\displaystyle =\displaystyle 0Multiply both sides by x\left(x + 3\right), with the condition x \neq0, -3
\displaystyle x\displaystyle =\displaystyle 5Solve for x

We know that the widths of the coops are given by x and x + 3. So the width of the smaller coop will be 5 feet, while the width of the larger coop will be x = 8 feet.

The length of each coop is the same, and we know that the length is equal to \dfrac{60}{x} from when we set up the equation. So the length of the two coops will be \dfrac{60}{5} = 12 feet.

In summary, one coop will be 12 \times 5 feet and the other will be 12 \times 8 feet.

Reflection

Since the value of x that we solved for is not 0 or -3, we know that it is not an extraneous solution.

Also, since x represents a width, it must be a positive value, which it is. If the solution had been negative, it would have been a non-viable solution.

Outcomes

MA.912.AR.1.8

Rewrite a polynomial expression as a product of polynomials over the real or complex number system.

MA.912.AR.1.9

Apply previous understanding of rational number operations to add, subtract, multiply and divide rational algebraic expressions.

MA.912.AR.8.1

Write and solve one-variable rational equations. Interpret solutions as viable in terms of the context and identify any extraneous solutions.

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