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Secondary 6 · Mathematics
Equations · Algebra

Operations with Algebraic Fractions

Two pipes fill the water tank of a corner store: one fills it alone in $x$ hours and the other in $x+2$ hours. What fraction of the tank do they fill together in one hour? The answer is a sum of fractions with $x$ in the denominators. An algebraic fraction is handled just like $\dfrac{10}{15}$: to simplify it, add it, or divide it, you have to factor first, just as with numbers you need to know their prime factors. You will need this tool in rational equations and in limits.

Simplifying by Factoring

Simplifying an algebraic fraction means canceling the factors that the numerator and denominator have in common. You always factor first: without factoring there is nothing to cancel. Remember the special products and factoring by cases, because they are all used here.

Step-by-step simplification of the algebraic fraction (x²-9)/(x²+6x+9) by canceling the common factor x plus 3. x² − 9 x² + 6x + 9 → (x−3)(x+3) (x+3)² = x − 3 x + 3 the factor (x+3) is canceled

Every rule has its reason: canceling a factor means dividing the top and bottom by the same (nonzero) number, and that does not change the value of the fraction. That is why only factors are canceled, never terms being added.

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Simplify the following algebraic fraction as far as possible by factorising the numerator and the denominator: $\frac{x^2 - 9}{x^2 + 6x + 9}$

Simplify the following algebraic fraction as far as possible by factorising the numerator and the denominator:

$\frac{x^2 - 9}{x^2 + 6x + 9}$

Solve

Multiplication and Division

Just as with numerical fractions: multiplying is numerator times numerator and denominator times denominator; dividing is multiplying by the reciprocal fraction. If you factor before multiplying, you can cancel on the fly without expanding anything.

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Perform the following multiplication of algebraic fractions and express the result in its simplest form: $\frac{2x}{x-1} \times \frac{x^2 - 1}{4x^2}$

Perform the following multiplication of algebraic fractions and express the result in its simplest form:

$\frac{2x}{x-1} \times \frac{x^2 - 1}{4x^2}$

Solve
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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:

$\frac{x^2 + 2x}{x - 3} : \frac{x^2 - 4}{x^2 - 9}$

Solve

Addition and Subtraction: The Least Common Multiple

With the same denominator, you add the numerators and the denominator does not change. With different denominators, you need a common denominator: factor each denominator and take the least common multiple (LCM), with the common and non-common factors at their highest exponent.

Subtraction of algebraic fractions by calculating the least common multiple of the factored denominators. 5/(x+2) − 3/(x²−4) x²−4 = (x−2)(x+2) → m.c.m. = (x−2)(x+2) = [5(x−2) − 3] / [(x−2)(x+2)] = (5x−13)/(x²−4)
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Add the following algebraic fractions with common denominators: $\frac{3x - 1}{x^2 + 1} + \frac{x + 5}{x^2 + 1}$

Add the following algebraic fractions with common denominators:

$\frac{3x - 1}{x^2 + 1} + \frac{x + 5}{x^2 + 1}$

Solve
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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:

$\frac{5}{x+2} - \frac{3}{x^2 - 4}$

Solve
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Add the following algebraic fractions with linear denominators: $\frac{x}{x-2} + \frac{2}{x+3}$

Add the following algebraic fractions with linear denominators:

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

Solve

Degree-3 Denominators: Synthetic Division

Sometimes the denominator is a degree-3 polynomial that is not an obvious special product. Look for an integer root among the divisors of the constant term and use synthetic division (Ruffini's rule), as in Secondary Year 3. If $x=a$ is a root, $(x-a)$ is a factor.

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Perform the addition and subtraction of the following algebraic fractions. Use Ruffini's rule and algebraic identities to factorise the denominators first and find the…

Perform the addition and subtraction of the following algebraic fractions. Use Ruffini's rule and algebraic identities to factorise the denominators first and find the lowest common multiple:

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

Solve
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The base of a rectangular solar panel is given by the expression $b = \frac{x^3 - x}{x^2 + 2x + 1}$ meters, and its height by $h = \frac{x^2 + x}{x^2 - 2x + 1}$ meters.…
b = (x³-x)/(x²+2x+1) h = (x²+x)/(x²-2x+1)

The base of a rectangular solar panel is given by the expression $b = \frac{x^3 - x}{x^2 + 2x + 1}$ meters, and its height by $h = \frac{x^2 + x}{x^2 - 2x + 1}$ meters. Calculate the area of the solar panel, expressing the result as a single simplified algebraic fraction.

Solve
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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.

Solve

Combined Operations

When sums, differences, products, and parentheses are mixed, you first work out what is inside the parentheses and then follow the order of operations: multiplication and division before addition and subtraction.

Order of operations in a combined expression with algebraic fractions and parentheses. (x²−x)/(x+1) · (1/x + 1/(x−1)) 1the parentheses 2factor and multiply 3cancel = (2x−1)/(x+1)
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Evaluate the expression inside the brackets and then multiply to simplify the final result: $\frac{x^2 - x}{x+1} \times \left( \frac{1}{x} + \frac{1}{x-1} \right)$

Evaluate the expression inside the brackets and then multiply to simplify the final result:

$\frac{x^2 - x}{x+1} \times \left( \frac{1}{x} + \frac{1}{x-1} \right)$

Solve
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Carry out the following combined operation of algebraic fractions and simplify as far as possible: $\frac{x^2+x}{x^2-1} - \frac{1}{x-1}$

Carry out the following combined operation of algebraic fractions and simplify as far as possible:

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

Solve
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Perform the following combined operations respecting the order of operations and simplify the result as far as possible:…

Perform the following combined operations respecting the order of operations and simplify the result as far as possible:

$\left( \frac{x+1}{x-1} - \frac{x-1}{x+1} \right) : \frac{4x}{x^2 - 1}$

Solve

Proving Algebraic Equalities

To prove an equality between expressions, you work on just one side (the more complicated one) until you reach the other. You perform the fraction operations and simplify, and you note that it holds for the values of $x$ that do not make any denominator zero.

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Show that the following algebraic equality holds for any values of the variables $x$ and $y$ that do not make the denominators zero, by carrying out the operations and…

Show that the following algebraic equality holds for any values of the variables $x$ and $y$ that do not make the denominators zero, by carrying out the operations and simplifying the left-hand side:

$\left( \frac{x^2 - y^2}{x+y} \right) \times \left( \frac{x^3 + y^3}{x^2 - xy + y^2} \right) = x^2 - y^2$

Solve
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Carry out the indicated operations involving three variables and prove that the final result simplifies to a constant integer:…

Carry out the indicated operations involving three variables and prove that the final result simplifies to a constant integer:

$\frac{x^2}{(x-y)(x-z)} + \frac{y^2}{(y-x)(y-z)} + \frac{z^2}{(z-x)(z-y)}$

Solve

Complex Fractions

A complex fraction has fractions inside its numerator or its denominator. You reduce the numerator and the denominator, each, to a single fraction and divide at the end. It is an example case: it is not used as much as the other operations.

Solving a complex fraction by reducing the numerator and denominator to simple fractions. 1 − 1/x² 1 + 1/x = (x²−1)/x² (x+1)/x reduce to a common denominator top and bottom

If there are several levels (a continued fraction), you start at the deepest level and work up one level at a time.

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Simplify the following complex algebraic fraction by first simplifying the numerator and denominator independently: $\frac{1 - \frac{1}{x^2}}{1 + \frac{1}{x}}$

Simplify the following complex algebraic fraction by first simplifying the numerator and denominator independently:

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

Solve
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Simplify the following continued fraction expression by working from the bottom up until an irreducible fraction is obtained:…

Simplify the following continued fraction expression by working from the bottom up until an irreducible fraction is obtained:

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

Solve
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A motorboat travels along a canal from a jetty to a buoy at a speed $v+w$ (where $w$ is the downstream current speed) and returns at a speed $v-w$ (upstream). The…

A motorboat travels along a canal from a jetty to a buoy at a speed $v+w$ (where $w$ is the downstream current speed) and returns at a speed $v-w$ (upstream). The distance between the points is $d$. Obtain the simplified expression for the average speed of the round trip, given by $v_m = \frac{2d}{\frac{d}{v+w} + \frac{d}{v-w}}$, and check whether it is equivalent to the expression $\frac{v^2 - w^2}{v}$.

Solve

Teamwork and Sums of Reciprocals

Let's go back to the tank from the beginning. If a pipe fills the tank in $x$ hours, in one hour it fills $\dfrac1x$ of the tank. The portions per hour add up.

The same structure appears whenever "rates per hour" are added: two pumps emptying a well, two crews painting a house, two machines packing mangoes. You add the reciprocals of the times, simplify, and flip at the end.

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Two industrial 3D printers manufacture a batch of parts working together. 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 together. The first printer takes $x$ hours to complete the job alone, and the second takes $x+3$ hours. The fraction of the work they complete together in one hour is the sum of their individual work rates. Write down and simplify the algebraic fraction representing that fraction of work per hour.

Solve

Averages of Fractions

Some problems have several chained operations. For the average of two quantities given by fractions, you add them and multiply by $\tfrac12$. This is what happens, for example, when mixing equal volumes of two juices or two solutions: the final concentration is the mean of the two.

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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 $C_m$.

Solve

Identities and the Difference Quotient

Two more uses of algebraic fractions.

Decomposing a fraction. To find $A$, $B$, $C$ in an identity such as $\dfrac{x^2+x+4}{(x+1)(x^2+1)}=\dfrac{A}{x+1}+\dfrac{Bx+C}{x^2+1}$, you bring the right side to a common denominator and set the numerators equal: two polynomials are identical if they have the same coefficients.

Difference quotient. For a given $f$, the expression $\dfrac{f(x+h)-f(x)}{h}$ is the average rate of change between $x$ and $x+h$. It is the basis of the derivative. It is calculated by performing the fraction operations and simplifying $h$.

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Determine the numerical values of the parameters $A$, $B$ and $C$ such that the following partial fraction decomposition identity holds for all $x$:…

Determine the numerical values of the parameters $A$, $B$ and $C$ such that the following partial fraction decomposition identity holds for all $x$:

$\frac{5x^2 - x + 2}{x^3 - x^2 + x - 1} = \frac{A}{x-1} + \frac{Bx+C}{x^2+1}$

Solve
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The surface area of a cylindrical container designed to minimise the use of plastic material is given by the expression $S(x) = 2\pi x^2 + \frac{1000}{x}$. To analyse…

The surface area of a cylindrical container designed to minimise the use of plastic material is given by the expression $S(x) = 2\pi x^2 + \frac{1000}{x}$. To analyse the rate of change of 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 $h$.

Solve

Summary

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