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Secondary 3 · Mathematics
Polynomials · Polynomials

Polynomial Division

Before you start, make sure you know: Operations with polynomials (Secondary Year 2 (first cycle))
Before you start, make sure you know: Special products (Secondary Year 2 (first cycle))

Yohanna and her friends are going to set up a small recording studio for bachata in a rectangular space. The area of the space is $3x^2 + 15x$ square meters and the width is $3x$ meters. How long is it? To find out, you have to divide polynomials. It works just like the division of numbers you learned in elementary school, and it also has a very fast shortcut: synthetic division (Ruffini's rule).

Dividing polynomials

By a monomial: divide each term of the polynomial by the monomial.

$$\frac{3x^2 + 15x}{3x} = \frac{3x^2}{3x} + \frac{15x}{3x} = x + 5$$

By a polynomial: it is done just like with numbers, term by term. Look at how $x^3 - 5x^2 + 3x - 4$ is divided by $x - 2$:

Step-by-step division of the polynomial x³ − 5x² + 3x − 4 by x − 2, with quotient x² − 3x − 3 and remainder −10. x³ − 5x² + 3x − 4 x − 2 x² − 3x − 3 −x³ + 2x² −3x² + 3x 3x² − 6x −3x − 4 3x − 6 −10 1. x³ : x = x² (to the quotient) 2. x² · (x − 2), with signs changed 3. add and bring down the next term 4. repeat until the degree is less than the divisor's remainder = −10
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Divide the following polynomial by the given monomial, simplifying each of the resulting terms: $$(12x^4 - 8x^3 + 4x^2) : (4x^2)$$

Divide the following polynomial by the given monomial, simplifying each of the resulting terms:

$$(12x^4 - 8x^3 + 4x^2) : (4x^2)$$
Solve
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The area of a rectangular recording studio is given by the polynomial $A(x) = 3x^2 + 15x$. If the width of the studio measures $3x$ meters, what is the algebraic…

The area of a rectangular recording studio is given by the polynomial $A(x) = 3x^2 + 15x$. If the width of the studio measures $3x$ meters, what is the algebraic expression that represents the length of the studio?

RECORDING STUDIO Area = 3x² + 15x 3x Length = ?
Solve
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Use polynomial long division to find the quotient $C(x)$ and the remainder $R(x)$ of the following polynomial operation. Remember to order and complete the terms of the…

Use polynomial long division to find the quotient $C(x)$ and the remainder $R(x)$ of the following polynomial operation. Remember to order and complete the terms of the dividend before starting:

$(6x^4 + 7x^3 - 5x^2 + 1) : (3x^2 - x + 2)$

Solve

Checking the division

This equality is used to check a division and also to rebuild the dividend. In the previous example: $(2x - 1)(3x^2 + x - 2) + 2 = 6x^3 - x^2 - 5x + 2 + 2 = 6x^3 - x^2 - 5x + 4$. It matches!

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In any polynomial division, the relation $P(x) = d(x) \times c(x) + r(x)$ holds. If when dividing a polynomial by $d(x) = x + 3$ we have obtained a quotient of…

In any polynomial division, the relation $P(x) = d(x) \times c(x) + r(x)$ holds. If when dividing a polynomial by $d(x) = x + 3$ we have obtained a quotient of $c(x) = x - 1$ and a remainder of $r(x) = 5$, what is the dividend polynomial $P(x)$?

Solve
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Reconstruct the dividend polynomial $D(x)$ knowing that when divided by $d(x) = x^2 - 1$, the quotient obtained is $C(x) = x + 5$ and the remainder is $R(x) = 3x - 2$.…

Reconstruct the dividend polynomial $D(x)$ knowing that when divided by $d(x) = x^2 - 1$, the quotient obtained is $C(x) = x + 5$ and the remainder is $R(x) = 3x - 2$. Use the fundamental property of division: $D(x) = d(x) \times C(x) + R(x)$.

Solve
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When dividing a polynomial $P(x)$ by $Q(x) = x^2 - 4$, we obtain a quotient of $C(x) = x^2 + 2x - 3$ and a remainder of $R(x) = 5x + 1$. Reconstruct the original…

When dividing a polynomial $P(x)$ by $Q(x) = x^2 - 4$, we obtain a quotient of $C(x) = x^2 + 2x - 3$ and a remainder of $R(x) = 5x + 1$. Reconstruct the original polynomial $P(x)$ and express it in standard form (ordered from highest to lowest degree).

Solve

Special quotients

Just as there are special products, there are special quotients: exact divisions that can be done "from memory." They come from reading the special products backwards.

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A team of young programmers is designing a level for a video game where the volume of a supply crate is $V(x) = x^3 - 6x^2 + 12x - 8$. If the base of the crate has an…

A team of young programmers is designing a level for a video game where the volume of a supply crate is $V(x) = x^3 - 6x^2 + 12x - 8$. If the base of the crate has an area of $A(x) = x^2 - 4x + 4$, what is the expression for the height of the crate? What value does the height take if $x = 5$ units?

h(x) = ? h(5) = ? V(x) = x³ - 6x² + 12x - 8 A(x) = x² - 4x + 4
Solve
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A rectangular billboard is to be designed with a total surface area defined by $S(x) = x^4 - 16$. If one of its sides measures $L(x) = x^2 + 4$, calculate the polynomial…

A rectangular billboard is to be designed with a total surface area defined by $S(x) = x^4 - 16$. If one of its sides measures $L(x) = x^2 + 4$, calculate the polynomial that represents the other side. Next, state whether the area is divisible by $(x - 2)$ and justify your answer without carrying out long division.

Total Surface Area S(x) = x⁴ - 16 Divisible by (x - 2)? L(x) = x² + 4 Base Polynomial?
Solve

Synthetic division (Ruffini's rule)

When the divisor has the form $x - a$, there is a shortcut that uses only the coefficients. Let's divide $x^3 - 5x^2 + 3x - 4$ by $x - 2$ again (so $a = 2$):

Diagram of Ruffini's rule for dividing x³ − 5x² + 3x − 4 by x − 2, with quotient x² − 3x − 3 and remainder −10. 1−53−4 2 2−6−6 1−3−3 −10 quotient: x² − 3x − 3remainder
  1. Write the coefficients of the dividend (with $0$ in the gaps) and the number $a$ on the left.

  2. Bring down the first coefficient.

  3. Multiply it by $a$, put the result under the next coefficient, and add.

  4. Repeat until the end. The last number is the remainder; the others are the coefficients of the quotient, which has one degree less than the dividend.

Why does it work? It's the same long division, but without writing the $x$'s: since the divisor is $x - a$, each step just multiplies by $a$ and adds.

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Apply Ruffini's rule to calculate the quotient and the remainder of the following division: $$(x^3 - 5x^2 + 3x + 9) : (x - 2)$$

Apply Ruffini's rule to calculate the quotient and the remainder of the following division:

$$(x^3 - 5x^2 + 3x + 9) : (x - 2)$$
Solve
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Use Ruffini's rule to divide the polynomial $P(x) = x^3 + 4x^2 - x - 4$ by the binomial $d(x) = x + 1$. Clearly state the quotient $C(x)$ and the remainder $R$.

Use Ruffini's rule to divide the polynomial $P(x) = x^3 + 4x^2 - x - 4$ by the binomial $d(x) = x + 1$. Clearly state the quotient $C(x)$ and the remainder $R$.

Solve
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Apply the Ruffini rule to calculate the quotient and the remainder of the division of the polynomial $P(x) = x^4 - 3x^2 + 5x - 10$ by the binomial $(x + 3)$. Remember…

Apply the Ruffini rule to calculate the quotient and the remainder of the division of the polynomial $P(x) = x^4 - 3x^2 + 5x - 10$ by the binomial $(x + 3)$.

Remember that, to apply the rule, the polynomial must be complete and ordered (if any term is missing, its coefficient is written as $0$) and that the value placed on the left is the root of the divisor.

  1. a) Write the coefficients of the dividend and the value of $a$ that you use in the Ruffini rule.
  1. b) Carry out the Ruffini rule step by step.
  1. c) Write the quotient polynomial $C(x)$ and clearly state the value of the remainder $R$.
Solve

When the divisor is not $x - a$

What if the divisor is $2x - 1$? We factor out the coefficient of $x$: $2x - 1 = 2\left(x - \tfrac{1}{2}\right)$. First divide by $x - \tfrac12$ using Ruffini ($a = \tfrac12$) and then divide the quotient by $2$. The remainder does not change.

Why? If $D = \left(x - \tfrac12\right)Q + R$, then $D = (2x - 1)\cdot\dfrac{Q}{2} + R$.

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Calculate the quotient and the remainder of the following division using Ruffini's Rule. Pay attention, as the divisor does not have a leading coefficient equal to 1:…

Calculate the quotient and the remainder of the following division using Ruffini's Rule. Pay attention, as the divisor does not have a leading coefficient equal to 1:

$$(4x^3 - 6x^2 + 8x - 5) : (2x - 1)$$

Remember that to use Ruffini's rule, the divisor must be of the form $(x - a)$. How could you transform $2x - 1$ to give it that form?

Solve

Problems with areas and volumes

Dividing polynomials is useful for finding an unknown measurement: if $\text{Area} = \text{length} \times \text{width}$, then $\text{length} = \text{Area} : \text{width}$. Volume works the same way: $V = (\text{area of the base}) \times \text{height}$.

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The area of a rectangular padel court is given by the polynomial $A(x) = 2x^2 + 11x + 12$. If the width of the court measures $(x + 4)$ meters, determine the algebraic…

The area of a rectangular padel court is given by the polynomial $A(x) = 2x^2 + 11x + 12$. If the width of the court measures $(x + 4)$ meters, determine the algebraic expression that represents its length.

Court area A(x) = 2x² + 11x + 12 x + 4 Length = ?
Solve
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A logistics company uses cardboard boxes whose volume in cubic centimeters is given by the polynomial $V(x) = 2x^3 + 9x^2 + 7x - 6$. If the base area of the box is…

A logistics company uses cardboard boxes whose volume in cubic centimeters is given by the polynomial $V(x) = 2x^3 + 9x^2 + 7x - 6$. If the base area of the box is $B(x) = 2x^2 + 5x - 3$, calculate the polynomial that represents the height $H(x)$ of the box.

H(x) = ? B(x) = 2x² + 5x - 3 V(x) = 2x³ + 9x² + 7x - 6
Solve
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The total profit of a video game store from the sale of a new release is expressed by the polynomial $B(x) = x^3 + 15x^2 + 54x$. If the profit for each video game sold…

The total profit of a video game store from the sale of a new release is expressed by the polynomial $B(x) = x^3 + 15x^2 + 54x$. If the profit for each video game sold is represented by the binomial $(x + 6)$, perform the division to find the polynomial that represents the total number of video games sold.

Solve

The remainder theorem

In the example at the beginning, the remainder of dividing by $x - 2$ was $-10$. Compute $P(2) = 8 - 20 + 6 - 4 = -10$. It matches! This is no coincidence:

This way you can find the remainder without dividing, just by substituting.

Diagram of the remainder theorem: the remainder of dividing P(x) by x − a is P(a), and if it equals 0, then a is a root. remainder of P(x) : (x − a) = P(a) if it equals 0 a is a root
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Without carrying out long division or using Ruffini's rule, calculate the value of the remainder of the following operation using the Remainder Theorem: $$(x^4 - 3x^2 +…

Without carrying out long division or using Ruffini's rule, calculate the value of the remainder of the following operation using the Remainder Theorem:

$$(x^4 - 3x^2 + 5x - 2) : (x - 1)$$
Solve
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Calculate the value of the letter $k$ so that the division is exact (that is, the remainder is $0$): $$(x^2 - 6x + k) : (x - 4)$$

Calculate the value of the letter $k$ so that the division is exact (that is, the remainder is $0$):

$$(x^2 - 6x + k) : (x - 4)$$
Solve
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Without carrying out the division, use the Remainder Theorem to find out whether the number $x = 2$ is a root of the polynomial $P(x) = 2x^3 - 5x^2 - x + 6$. Justify…

Without carrying out the division, use the Remainder Theorem to find out whether the number $x = 2$ is a root of the polynomial $P(x) = 2x^3 - 5x^2 - x + 6$. Justify your answer by indicating whether the division of $P(x)$ by $(x - 2)$ has no remainder (is exact) or not.

Solve

Finding parameters with the remainder theorem

When a polynomial has an unknown letter ($k$, $a$, $b$), the remainder theorem lets you set up an equation with it: substitute $x = a$ and set it equal to the remainder you are given.

If there are two letters, you need two conditions and you get a system of equations. For example, if $P$ is divisible by $x - 1$ and by $x + 2$, then $P(1) = 0$ and $P(-2) = 0$.

If you compare the remainders of two divisions, you compute each remainder with the theorem and set them equal.

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Find the value of the parameter $k$ so that the division of the polynomial $P(x) = x^3 - kx^2 + 4x - 8$ by the binomial $(x - 2)$ has a remainder equal to $10$.

Find the value of the parameter $k$ so that the division of the polynomial $P(x) = x^3 - kx^2 + 4x - 8$ by the binomial $(x - 2)$ has a remainder equal to $10$.

Solve
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Find the values of the parameters $a$ and $b$ so that the polynomial $P(x) = x^4 - ax^3 + bx^2 + x - 6$ is divisible simultaneously by $(x - 1)$ and by $(x + 2)$. Hint:…

Find the values of the parameters $a$ and $b$ so that the polynomial $P(x) = x^4 - ax^3 + bx^2 + x - 6$ is divisible simultaneously by $(x - 1)$ and by $(x + 2)$. Hint: use the Remainder Theorem to set up a system of equations.

Solve
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Determine the value of $k$ so that the remainder of the division $(2x^4 - 5x^3 + kx^2 - 12) : (x + 2)$ is equal to the remainder of the division…

Determine the value of $k$ so that the remainder of the division $(2x^4 - 5x^3 + kx^2 - 12) : (x + 2)$ is equal to the remainder of the division $(x^3 + 2x^2 - x + 10) : (x - 3)$.

Solve

Roots and factoring

A number $a$ is a root or zero of $P(x)$ if $P(a) = 0$. By the remainder theorem, this is equivalent to the division by $(x - a)$ being exact, that is, $(x - a)$ being a factor of $P(x)$.

Factoring means writing a polynomial as a product of simpler polynomials. For those of degree greater than 2 you need two tools that tell you where to look for the roots.

The roots are the points where the graph crosses the $x$-axis. Try it with this polynomial:

Monic polynomials

For example, $x^2 - 3x + 2$ is monic, but $2x^2 + 5x - 3$ is not because its leading coefficient is $2$. When dividing by $(x - a)$ with Ruffini, the first coefficient of the quotient is always equal to that of the dividend: if the dividend is monic, so is the quotient.

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Check whether the number $a = 3$ is a root of the polynomial $P(x) = x^3 - 3x^2 + x - 3$ by carrying out division using Ruffini's rule and observing the remainder…

Check whether the number $a = 3$ is a root of the polynomial $P(x) = x^3 - 3x^2 + x - 3$ by carrying out division using Ruffini's rule and observing the remainder obtained. Justify your answer based on the result of the operation.

Solve
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Check whether the binomial $(x + 1)$ is a factor of the polynomial $P(x) = x^4 - 1$. To do this, you can use long division, Ruffini's rule, or the Factor Theorem.…

Check whether the binomial $(x + 1)$ is a factor of the polynomial $P(x) = x^4 - 1$. To do this, you can use long division, Ruffini's rule, or the Factor Theorem. Explain what it means for a binomial to be a "factor" of another polynomial in terms of the remainder of the division.

Solve
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Consider the polynomial $P(x) = 3x^3 - kx^2 + 12x - 4$. 1. Calculate the value of $k$ so that $x = 2$ is a root of the polynomial. 2. Once $k$ is found, use Ruffini's…

Consider the polynomial $P(x) = 3x^3 - kx^2 + 12x - 4$.

  1. Calculate the value of $k$ so that $x = 2$ is a root of the polynomial.

  2. Once $k$ is found, use Ruffini's Rule to divide $P(x)$ by $(x - 2)$ and check that the remainder is zero.

  3. Is the quotient obtained a monic quadratic polynomial? (A polynomial is monic if its leading coefficient is 1).

Solve

Back to the beginning: the length of Yohanna's studio is $x + 5$ meters, because $\dfrac{3x^2 + 15x}{3x} = x + 5$.

Summary

See all the exercises in this section