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Equations · Equations

Rational Equations

Before you start, make sure you know: Algebraic fractions (High School Year 1 (semesters 1 and 2))
Before you start, make sure you know: Factoring and synthetic division (High School Year 1 (semesters 1 and 2))

A group of high school friends rents a bus to go to Teotihuacan and splits the cost. If five more people suddenly join, each one has to pay less. To find out how many there were at the start, the unknown ends up in a denominator: that is a rational equation. Here you will learn how to solve them and how to avoid the trap of false solutions.

Domain and the basic method

Rational equation with denominators that become zero at x = 2 and x = -2, showing that they are excluded from the domain. x/(x−2) + 2x/(x+2) = 16/(x²−4) denominators = 0 when x = 2 or x = −2 those values are always outside the domain
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Solve the rational equation by first putting it over a common denominator: $\frac{1}{x} + \frac{1}{2x} + \frac{1}{3x} = \frac{11}{6}$

Solve the rational equation by first putting it over a common denominator:

$\frac{1}{x} + \frac{1}{2x} + \frac{1}{3x} = \frac{11}{6}$

Solve
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Solve the following rational equation, clearly indicating its valid solutions: $\frac{3}{x} - \frac{1}{x+2} = \frac{5}{x(x+2)}$

Solve the following rational equation, clearly indicating its valid solutions:

$\frac{3}{x} - \frac{1}{x+2} = \frac{5}{x(x+2)}$

Solve
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Determine the solution to the equation, paying attention to the values that make the denominators zero: $\frac{2x}{x+1} + \frac{1}{x-1} = 2$

Determine the solution to the equation, paying attention to the values that make the denominators zero:

$\frac{2x}{x+1} + \frac{1}{x-1} = 2$

Solve

Factoring the denominators before finding the LCM

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Find the values of $x$ that satisfy the following equality: $\frac{x+3}{x-1} = \frac{x-2}{x+1}$

Find the values of $x$ that satisfy the following equality:

$\frac{x+3}{x-1} = \frac{x-2}{x+1}$

Solve
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Determine the real number $x$ that satisfies the equality between these two algebraic fractions: $\frac{x-1}{x} = \frac{x}{x+3}$

Determine the real number $x$ that satisfies the equality between these two algebraic fractions:

$\frac{x-1}{x} = \frac{x}{x+3}$

Solve
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Find the solutions to the following equation: $\frac{4}{x-2} - \frac{3}{x+2} = \frac{1}{x^2-4}$

Find the solutions to the following equation:

$\frac{4}{x-2} - \frac{3}{x+2} = \frac{1}{x^2-4}$

Solve
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Solve the equation by simplifying the algebraic fractions: $\frac{x+1}{x-1} - \frac{x-1}{x+1} = \frac{4}{x^2-1}$

Solve the equation by simplifying the algebraic fractions:

$\frac{x+1}{x-1} - \frac{x-1}{x+1} = \frac{4}{x^2-1}$

Solve
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Solve the rational equation by first factorising the denominators to find the lowest common multiple: $\frac{x}{x^2-4} + \frac{1}{x+2} = \frac{3}{x-2}$

Solve the rational equation by first factorising the denominators to find the lowest common multiple:

$\frac{x}{x^2-4} + \frac{1}{x+2} = \frac{3}{x-2}$

Solve

Watch out for extraneous solutions

Diagram of the solving process showing how to discard a candidate solution when it matches a value excluded from the domain. transformed equation x = 3, x = −7 x = 3makes a denominator zero⇒ discarded ✗ x = −7is a solution ✓
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Calculate the real values of $x$ for which the following expression holds: $\frac{x^2-9}{x-3} = 6$

Calculate the real values of $x$ for which the following expression holds:

$\frac{x^2-9}{x-3} = 6$

Solve
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Solve the following rational equation: $\frac{2}{x+3} + \frac{x}{x-3} = 1$

Solve the following rational equation:

$\frac{2}{x+3} + \frac{x}{x-3} = 1$

Solve
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Find the solutions to the following equation, discarding any that turn out to be false (extraneous solutions): $\frac{2x+1}{x^2-x-6} - \frac{x}{x-3} = \frac{1}{x+2}$

Find the solutions to the following equation, discarding any that turn out to be false (extraneous solutions):

$\frac{2x+1}{x^2-x-6} - \frac{x}{x-3} = \frac{1}{x+2}$

Solve
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Find the values of $x$ that satisfy the following equation: $\frac{x^2-x}{x^2-4x+3} = \frac{2x}{x-3} - 1$

Find the values of $x$ that satisfy the following equation:

$\frac{x^2-x}{x^2-4x+3} = \frac{2x}{x-3} - 1$

Solve

Problems with rational equations

Concept diagram relating combined-work and speed problems to rational equations where time appears in the denominator. combined work1/A + 1/B = 1/together speedtime = distance / speed the unknown ends up in a denominatorwhen setting up the time

With speeds and prices the pattern is similar: time is $t=\dfrac{d}{v}$ and the payment per person is $\dfrac{\text{total}}{n}$. Both have the unknown in the denominator.

This is how the opening story ends: if the bus costs $\$8{,}000$ and with $5$ more people each one pays $\$80$ less, then with $n$ original people we set up $\dfrac{8000}{n}-\dfrac{8000}{n+5}=80$. Multiplying by $n(n+5)$ and dividing by $80$ gives $n^2+5n-500=0$, so $n=20$ (we discard $n=-25$). There were 20 students.

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Two 3D printers working together take $6$ hours to produce a full batch of parts for an engineering project. If they worked separately, printer A would take $5$ hours…

Two 3D printers working together take $6$ hours to produce a full batch of parts for an engineering project. If they worked separately, printer A would take $5$ hours less than printer B to complete the same batch. Set up a rational equation and determine how long printer A would take working alone.

Solve
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A freight train must cover a distance of $300\text{ km}$. If it increased its average speed by $10\text{ km/h}$, it would take $1$ hour less to complete the whole…

A freight train must cover a distance of $300\text{ km}$. If it increased its average speed by $10\text{ km/h}$, it would take $1$ hour less to complete the whole journey. Set up a rational equation and calculate the average speed at which the train initially travels.

Solve
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A group of students rents a bus for a field trip for a total price of $8\,000\text{ \$}$. At the last minute, $5…

A group of students rents a bus for a field trip for a total price of $8\,000\text{ \$}$. At the last minute, $5$ more students sign up for the trip, which means that each of those who were originally going pays $80\text{ \$}$ less. Set up a rational equation to calculate how many students were in the original group.

Solve

Currents, mixtures, and areas

There are three more situations that always end up with the unknown on the bottom. In a current problem, the actual speed adds to or subtracts from the river's speed. In a mixture problem, the concentration is $\dfrac{\text{amount of salt}}{\text{volume}}$ and the volume changes. In a rectangle, one side is $\dfrac{\text{area}}{\text{other side}}$.

For the rectangular poster with area $600$ cm², the height is $\dfrac{600}{b}$ if the base is $b$. When the dimensions change, the area stays the same: $(b+10)\left(\dfrac{600}{b}-5\right)=600$.

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A graphic designer is creating a rectangular poster whose area must be exactly $600\text{ cm}^2$. If he increased the base of the poster by $10\text{ cm}$ and reduced…

A graphic designer is creating a rectangular poster whose area must be exactly $600\text{ cm}^2$. If he increased the base of the poster by $10\text{ cm}$ and reduced the height by $5\text{ cm}$, the total area would remain the same. Set up a rational equation and determine the original dimensions of the poster.

x 600/x Area = 600 A x + 10 600/x - 5 Area = 600 B
Solve
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A motorboat travels along a river, covering $24\text{ km}$ downstream and then returning to the starting point upstream. The total navigation time was $5\text{ hours}$.…

A motorboat travels along a river, covering $24\text{ km}$ downstream and then returning to the starting point upstream. The total navigation time was $5\text{ hours}$. If the speed of the river current is $2\text{ km/h}$, set up a rational equation to calculate the speed of the boat in still water.

Solve
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In a laboratory, there are $50\text{ L}$ of a saline solution with an initial concentration of $0.04\text{ kg/L}$. Water is to be evaporated from the mixture at a…

In a laboratory, there are $50\text{ L}$ of a saline solution with an initial concentration of $0.04\text{ kg/L}$. Water is to be evaporated from the mixture at a constant rate to increase its concentration (the amount of salt remains unchanged). If $x\text{ L}$ of water is evaporated, the new concentration must be $0.1\text{ kg/L}$. Set up the corresponding rational equation and calculate the amount of water that must be evaporated.

Solve

Similar triangles and the cable formula

Two vertical poles are parallel, so when a cable is stretched from the top of each one to the base of the other, triangles with the same angles are formed. This lets us derive the relationship $\dfrac1h=\dfrac1{h_1}+\dfrac1{h_2}$ given in the exercise.

Two vertical poles of heights h1 and h2 with two crossing cables that meet at a height h above the ground. h₁ h₂ h p d − p

Call $d$ the distance between the poles and $p$ the distance from pole 1 to the point below the crossing. With the triangle of the cable that leaves from the top of pole 2: $\dfrac{h}{h_2}=\dfrac{p}{d}$. With the other cable: $\dfrac{h}{h_1}=\dfrac{d-p}{d}$.

Adding: $\dfrac{h}{h_1}+\dfrac{h}{h_2}=\dfrac{d-p}{d}+\dfrac{p}{d}=1$. Dividing by $h$ gives $\dfrac1{h_1}+\dfrac1{h_2}=\dfrac1h$.

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Two vertical poles of heights $x$ and $x+2$ meters are separated by a certain distance on flat ground. Two straight, taut cables are stretched from the top of each pole…

Two vertical poles of heights $x$ and $x+2$ meters are separated by a certain distance on flat ground. Two straight, taut cables are stretched from the top of each pole to the base of the opposite pole. The point where the cables cross is at a height of $2.4\text{ m}$ from the ground. Knowing that the crossing height $h$ satisfies the geometric relation $\frac{1}{h} = \frac{1}{h_1} + \frac{1}{h_2}$ (where $h_1$ and $h_2$ are the heights of the poles), set up a rational equation and calculate the height of both poles.

Solve
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In an electronic circuit, two resistors are connected in parallel. The total equivalent resistance, $R_{eq}$, is calculated using the formula…

In an electronic circuit, two resistors are connected in parallel. The total equivalent resistance, $R_{eq}$, is calculated using the formula $\frac{1}{R_{eq}} = \frac{1}{R_1} + \frac{1}{R_2}$. It is known that the value of resistance $R_2$ is $5\ \Omega$ greater than that of $R_1$, and that the equivalent resistance of the circuit is $6\ \Omega$. Set up and solve a rational equation to find the value of both resistors.

A B R₁ = x R₂ = x+5 Eq. 6Ω
Solve

Advanced equations and equations with a parameter

Some equations are simplified with a special product (for example, $\dfrac{x^3-8}{x-2}=x^2+2x+4$) or with a change of variable (for example, $t=\dfrac{x^2+1}{x}$, which turns the equation into a quadratic in $t$; afterward you go back to $x$).

Diagram for discussing a rational equation with a parameter, branching into infinitely many solutions restricted to the domain or no solution. after simplifying:2x² + 2a = 2x² + 2 a = 1identity:infinitely many solutions a ≠ 1no solution
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Solve the following rational equation, first factorising the denominators to operate with the lowest common multiple:…

Solve the following rational equation, first factorising the denominators to operate with the lowest common multiple:

$\frac{1}{x^2-5x+6} + \frac{1}{x^2-7x+12} = \frac{2}{x^2-6x+8}$

Solve
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Solve the following rational equation by applying the substitution $t = \frac{x^2+1}{x}$: $\frac{x^2+1}{x} + \frac{6x}{x^2+1} = 5$

Solve the following rational equation by applying the substitution $t = \frac{x^2+1}{x}$:

$\frac{x^2+1}{x} + \frac{6x}{x^2+1} = 5$

Solve
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Determine all the real solutions to the equation, simplifying the algebraic fractions using algebraic identities before operating:…

Determine all the real solutions to the equation, simplifying the algebraic fractions using algebraic identities before operating:

$\frac{x^3-8}{x-2} = \frac{x^3+1}{x+1} + 3$

Solve
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Given the rational equation with parameter $a$: $\frac{x+a}{x-1} + \frac{x-a}{x+1} = \frac{2x^2+2}{x^2-1}$ Determine for which values of parameter $a$ the equation has…

Given the rational equation with parameter $a$:

$\frac{x+a}{x-1} + \frac{x-a}{x+1} = \frac{2x^2+2}{x^2-1}$

Determine for which values of parameter $a$ the equation has infinitely many solutions, and for which values it has no solution.

Solve

Summary

See all the exercises in this section