Operations with Rational Expressions
You already know that $\dfrac{10}{15}$ simplifies to $\dfrac{2}{3}$ because both numbers share the factor $5$. Can you do the same with $\dfrac{x^2-9}{x^2+6x+9}$? And does $\dfrac{5}{x+2}-\dfrac{3}{x^2-4}$ behave like $\dfrac12-\dfrac13$? It does. A rational expression is a fraction whose numerator and denominator are polynomials, and it follows the same rules as numeric fractions. The one new skill is factoring, which plays the role that prime factors play for numbers.
Simplifying by Factoring
To simplify a rational expression, factor the numerator and the denominator completely, then cancel the factors they share. Without factoring there is nothing to cancel.
Here is your factoring toolbox. You will use it on every page of this section.
| Pattern | Factored form |
|---|---|
| Greatest common factor | $6x^2+9x = 3x(2x+3)$ |
| Difference of squares | $a^2-b^2=(a-b)(a+b)$ |
| Perfect square trinomial | $a^2\pm 2ab+b^2=(a\pm b)^2$ |
| Perfect cube (cube of a binomial) | $a^3\pm 3a^2b+3ab^2\pm b^3=(a\pm b)^3$ |
| Sum of cubes | $a^3+b^3=(a+b)(a^2-ab+b^2)$ |
| Difference of cubes | $a^3-b^3=(a-b)(a^2+ab+b^2)$ |
| Trinomial $x^2+bx+c$ | find two numbers that multiply to $c$ and add to $b$ |
The graphs agree with this. The two expressions below trace the same curve, except that the original has a hole at $x=-3$.
Simplify the following algebraic fraction completely by factoring the numerator and the denominator: $\frac{x^2 - 9}{x^2 + 6x + 9}$
Simplify the following algebraic fraction completely by factoring the numerator and the denominator:
$\frac{x^2 - 9}{x^2 + 6x + 9}$
Multiplying and Dividing
Multiply rational expressions the way you multiply fractions: numerators together, denominators together. To divide, multiply by the reciprocal of the divisor.
Why flip? Dividing by $\dfrac{C}{D}$ asks "how many $\dfrac{C}{D}$ fit in here?", and multiplying by $\dfrac{D}{C}$ does exactly that. Factor before you multiply so you can cancel as you go. If you multiply first, you create big polynomials that are hard to factor again.
Perform the following multiplication of algebraic fractions and express the result in its simplest form: $\frac{2x}{x-1} \cdot \frac{x^2 - 1}{4x^2}$
Perform the following multiplication of algebraic fractions and express the result in its simplest form:
$\frac{2x}{x-1} \cdot \frac{x^2 - 1}{4x^2}$
Divide the following algebraic fractions and simplify the result obtained: $\frac{x^2 + 2x}{x - 3} : \frac{x^2 - 4}{x^2 - 9}$
Divide the following algebraic fractions and simplify the result obtained:
$\frac{x^2 + 2x}{x - 3} : \frac{x^2 - 4}{x^2 - 9}$
Prove that the following algebraic equality is true for any value of the variables $x$ and $y$ that does not make the denominators zero, by expanding the operations and…
Prove that the following algebraic equality is true for any value of the variables $x$ and $y$ that does not make the denominators zero, by expanding the operations and simplifying the left-hand side:
$\left( \frac{x^2 - y^2}{x+y} \right) \cdot \left( \frac{x^3 + y^3}{x^2 - xy + y^2} \right) = x^2 - y^2$
Adding and Subtracting
With the same denominator, add or subtract the numerators and keep the denominator:
$$\frac{A}{D}\pm\frac{B}{D}=\frac{A\pm B}{D}$$With different denominators, rewrite each fraction over the least common denominator (LCD). To find it, factor every denominator and take each factor at its highest power.
Add the following algebraic fractions that have the same denominator: $\frac{3x - 1}{x^2 + 1} + \frac{x + 5}{x^2 + 1}$
Add the following algebraic fractions that have the same denominator:
$\frac{3x - 1}{x^2 + 1} + \frac{x + 5}{x^2 + 1}$
Subtract the following algebraic fractions by first finding a common denominator: $\frac{5}{x+2} - \frac{3}{x^2 - 4}$
Subtract the following algebraic fractions by first finding a common denominator:
$\frac{5}{x+2} - \frac{3}{x^2 - 4}$
Perform the following addition of algebraic fractions with first-degree denominators: $\frac{x}{x-2} + \frac{2}{x+3}$
Perform the following addition of algebraic fractions with first-degree denominators:
$\frac{x}{x-2} + \frac{2}{x+3}$
Bigger Denominators and Opposite Factors
Real problems have denominators with repeated factors, cubics, or factors that are opposites of each other. Here are three tools.
Repeated factors. Take the highest power of each factor. For $(x-2)^2$ and $(x-2)(x+2)$ the LCD is $(x-2)^2(x+2)$.
Opposite factors. $a-b=-(b-a)$. So $x-3$ and $3-x$ are opposites, and $\dfrac{3-x}{x-3}=-1$. When two denominators are opposites, rewrite one so the factors match:
$$\frac{1}{x-3}+\frac{2}{3-x}=\frac{1}{x-3}-\frac{2}{x-3}=\frac{-1}{x-3}$$Cubic denominators. First look for a pattern from the toolbox. A cubic with alternating signs like $x^3-3x^2+3x-1$ is a perfect cube. Otherwise, find a zero by trial, then use synthetic division to pull out a factor. Some problems call this Ruffini's rule; it is the same procedure.
Perform the addition and subtraction of the following algebraic fractions. Use Ruffini's rule and standard identities to first factor the denominators and find the least…
Perform the addition and subtraction of the following algebraic fractions. Use Ruffini's rule and standard identities to first factor the denominators and find the least common multiple:
$\frac{2x}{x^3 - 3x^2 + 3x - 1} - \frac{1}{x^2 - 2x + 1} + \frac{3}{x-1}$
The average production cost of an electronic component is given by the expression $C_m(x) = \frac{x^3 + 2x^2 - 5x - 6}{x^2 - 4}$, where $x$ is the production volume in…
The average production cost of an electronic component is given by the expression $C_m(x) = \frac{x^3 + 2x^2 - 5x - 6}{x^2 - 4}$, where $x$ is the production volume in thousands of units. Due to a subsidy adjustment, a fee represented by the fraction $\frac{x^2-1}{x+2}$ must be subtracted. Calculate the new expression for the average cost by first simplifying the first fraction.
Perform the indicated operations with three variables and show that the final result simplifies to a constant integer:…
Perform the indicated operations with three variables and show that the final result simplifies to a constant integer:
$\frac{x^2}{(x-y)(x-z)} + \frac{y^2}{(y-x)(y-z)} + \frac{z^2}{(z-x)(z-y)}$
Combined Operations
When an expression mixes parentheses, sums, products, and quotients, follow the order of operations exactly as with numbers: parentheses first, then multiplication and division from left to right, then addition and subtraction from left to right.
Work out the expression inside the brackets and then multiply to simplify the final result: $\frac{x^2 - x}{x+1} \cdot \left( \frac{1}{x} + \frac{1}{x-1} \right)$
Work out the expression inside the brackets and then multiply to simplify the final result:
$\frac{x^2 - x}{x+1} \cdot \left( \frac{1}{x} + \frac{1}{x-1} \right)$
Perform the following combined operation of algebraic fractions and simplify completely: $\frac{x^2+x}{x^2-1} - \frac{1}{x-1}$
Perform the following combined operation of algebraic fractions and simplify completely:
$\frac{x^2+x}{x^2-1} - \frac{1}{x-1}$
Perform the following combined operations respecting the order of operations and simplify the result completely:…
Perform the following combined operations respecting the order of operations and simplify the result completely:
$\left( \frac{x+1}{x-1} - \frac{x-1}{x+1} \right) : \frac{4x}{x^2 - 1}$
Complex Fractions
A complex fraction has fractions in its numerator, its denominator, or both. There are two reliable methods.
Method 1. Combine the top into one fraction and the bottom into one fraction, then divide (multiply by the reciprocal).
Method 2. Multiply the top and bottom of the whole thing by the LCD of all the small denominators. This clears every inner fraction in one step. It is legal because you are multiplying by $\dfrac{\text{LCD}}{\text{LCD}}=1$.
Letters other than $x$ work the same way. In a problem with $\dfrac{2d}{\frac{d}{v+w}+\frac{d}{v-w}}$, the letter $d$ is just another common factor to cancel. Factor it out of the denominator and watch it vanish.
Simplify the following complex algebraic fraction by first reducing the numerator and the denominator independently: $\frac{1 - \frac{1}{x^2}}{1 + \frac{1}{x}}$
Simplify the following complex algebraic fraction by first reducing the numerator and the denominator independently:
$\frac{1 - \frac{1}{x^2}}{1 + \frac{1}{x}}$
Simplify the following iterated algebraic expression (continued fraction) by working from the deepest level upward, until obtaining an irreducible fraction:…
Simplify the following iterated algebraic expression (continued fraction) by working from the deepest level upward, until obtaining an irreducible fraction:
$1 + \frac{1}{1 + \frac{1}{1 + \frac{1}{x-1}}}$
A delivery drone travels from a warehouse to a residential area at a speed of $v+w$ (where $w$ is the tailwind speed) and returns at a speed of $v-w$ (headwind). The…
A delivery drone travels from a warehouse to a residential area at a speed of $v+w$ (where $w$ is the tailwind speed) and returns at a speed of $v-w$ (headwind). The distance between the points is $d$. Obtain the simplified expression for the average speed of the entire journey, given by $v_m = \frac{2d}{\frac{d}{v+w} + \frac{d}{v-w}}$, and check if it is equivalent to the expression $\frac{v^2 - w^2}{v}$.
Formulas from Circuits and Work Rates
Many formulas in science and everyday life add reciprocals. In physics, two resistors in parallel have an equivalent resistance $R$ with
$$\frac{1}{R}=\frac{1}{R_1}+\frac{1}{R_2}$$The same pattern shows up with work rates: if a job takes you $t$ hours alone, you finish $\dfrac1t$ of it per hour, and rates add when people work together. The plan is always the same: add the fractions, simplify, then flip to get $R$ (or the time).
In an electrical circuit, two resistors $R_1$ and $R_2$ are connected in parallel. The equivalent resistance $R_{eq}$ satisfies the relationship…
In an electrical circuit, two resistors $R_1$ and $R_2$ are connected in parallel. The equivalent resistance $R_{eq}$ satisfies the relationship $\frac{1}{R_{eq}} = \frac{1}{R_1} + \frac{1}{R_2}$. If the value of the resistors is given by the polynomials $R_1 = x^2 - 4$ and $R_2 = x^2 + 4x + 4$, obtain the simplified expression for the equivalent resistance $R_{eq}$ in terms of $x$.
The thin lens formula in optics states that $\frac{1}{f} = \frac{1}{s_o} + \frac{1}{s_i}$. If the distance from the object to the optical center is…
The thin lens formula in optics states that $\frac{1}{f} = \frac{1}{s_o} + \frac{1}{s_i}$. If the distance from the object to the optical center is $s_o = \frac{x^2+x}{2}$ and the image distance is $s_i = \frac{x^2-1}{3}$, determine the simplified algebraic expression for the focal length $f$.
Two industrial 3D printers manufacture a batch of parts working as a team. The first printer takes $x$ hours to complete the job alone, and the second takes $x+3$ hours.…
Two industrial 3D printers manufacture a batch of parts working as a team. The first printer takes $x$ hours to complete the job alone, and the second takes $x+3$ hours. The portion of the work they do together in one hour is the sum of their individual work rates. Write and simplify the algebraic fraction that represents this portion of work per hour.
Building Expressions: Travel, Area, and Averages
Some problems don't hand you the fractions. You have to build them from the situation, then simplify. These formulas show up a lot:
-
$\text{time}=\dfrac{\text{distance}}{\text{rate}}$
-
Average of two quantities $=\dfrac12(\text{first}+\text{second})$
-
Average speed $=\dfrac{\text{total distance}}{\text{total time}}$ (not the average of the two speeds!)
The base of a rectangular solar panel is given by the expression $b = \frac{x^3 - x}{x^2 + 2x + 1}$ feet, and its height by $h = \frac{x^2 + x}{x^2 - 2x + 1}$ feet.…
The base of a rectangular solar panel is given by the expression $b = \frac{x^3 - x}{x^2 + 2x + 1}$ feet, and its height by $h = \frac{x^2 + x}{x^2 - 2x + 1}$ feet. Calculate the area of the solar panel, expressing the result as a single simplified algebraic fraction.
A mixed circuit has an equivalent resistance between terminals A and B given by the formula $R_{eq} = R_1 + \frac{1}{\frac{1}{R_2} + \frac{1}{R_3}}$. If the values of…
A mixed circuit has an equivalent resistance between terminals A and B given by the formula $R_{eq} = R_1 + \frac{1}{\frac{1}{R_2} + \frac{1}{R_3}}$. If the values of the resistors in ohms are given by the algebraic expressions $R_1 = \frac{x}{x+1}$, $R_2 = \frac{x^2-1}{x}$ and $R_3 = \frac{x^2-1}{2}$, calculate the simplified expression for the equivalent resistance $R_{eq}$.
In a chemistry laboratory, there are two solutions of the same acid. The first has a concentration of $C_1 = \frac{x}{x^2+2x+1}$ and the second $C_2 = \frac{2}{x^2-1}$.…
In a chemistry laboratory, there are two solutions of the same acid. The first has a concentration of $C_1 = \frac{x}{x^2+2x+1}$ and the second $C_2 = \frac{2}{x^2-1}$. Equal volumes of both solutions are mixed, so the concentration of the resulting mixture is the arithmetic mean of both concentrations, that is, $C_m = \frac{1}{2}(C_1 + C_2)$. Obtain the simplified algebraic expression for the final concentration of the mixture $C_m$.
Difference Quotients and Partial Fractions
Two kinds of rational-expression work come back in Precalculus and calculus.
The difference quotient. For a function $f$, the expression $\dfrac{f(x+h)-f(x)}{h}$ measures the average rate of change over a tiny step $h$. For rational functions you subtract two fractions, then divide by $h$. The key is that the numerator always contains a factor of $h$, which cancels.
Partial fraction decomposition runs addition backward. You split one fraction into a sum of simpler ones with unknown numerators. Then you make both sides equal as polynomials. You can plug in convenient values of $x$ or match coefficients.
The two curves lie on top of each other, so the decomposition is an identity. When the denominator has an irreducible quadratic factor like $x^2+1$, the numerator over it is allowed to be linear, $Bx+C$. Then it is easier to match coefficients of $x^2$, $x$, and the constant term.
Determine the numerical values of the parameters $A$, $B$ and $C$ so that the following partial fraction decomposition identity holds for all $x$:…
Determine the numerical values of the parameters $A$, $B$ and $C$ so that the following partial fraction decomposition identity holds for all $x$:
$\frac{5x^2 - x + 2}{x^3 - x^2 + x - 1} = \frac{A}{x-1} + \frac{Bx+C}{x^2+1}$
The surface area of a cylindrical container designed to minimize the use of plastic material is given by the expression $S(x) = 2\pi x^2 + \frac{1000}{x}$. To analyze…
The surface area of a cylindrical container designed to minimize the use of plastic material is given by the expression $S(x) = 2\pi x^2 + \frac{1000}{x}$. To analyze the rate of change of the material with respect to the radius $x$, an engineer needs to calculate and simplify the expression for the difference quotient $\frac{S(x+h) - S(x)}{h}$. Perform this operation and express the result as a single simplified algebraic fraction in terms of $x$ and $h$.
Summary
Back to the start: $\dfrac{x^2-9}{x^2+6x+9}$ simplifies to $\dfrac{x-3}{x+3}$ once you factor, and $\dfrac{5}{x+2}-\dfrac{3}{x^2-4}$ becomes $\dfrac{5x-13}{(x-2)(x+2)}$ over the LCD. Rational expressions behave like numeric fractions. The difference is that you must factor to see the common factors.