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Equations · Algebra

The Remainder Theorem

Before you start, make sure you know: Factoring and synthetic division (High School Year 1 (semesters 1 and 2))
Before you start, make sure you know: Systems of linear equations (High School Year 1 (semesters 1 and 2))

In a computer science class you are asked for the remainder of dividing $P(x)=x^{2024}-3x^{2023}+2x^5-7x+4$ by $x-3$. Nobody does that whole division: with a single substitution, $P(3)$, the problem is solved in seconds. The remainder theorem is that substitution. At the end of the page we come back to this problem.

The remainder theorem

Diagram identifying the remainder of dividing P(x) by x minus a with the numerical value P(a). remainder of P(x) ÷ (x−a) = P(a) just evaluate P at x = a, no division needed

A numerical value can also be interpreted this way: if $x(t)=-2t^3+8t^2-5t+3$ is the position of an object, $x(3)$ is the position at $t=3$ and, at the same time, the remainder of dividing $x(t)$ by $t-3$.

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Without performing the division, calculate the remainder when dividing the polynomial $P(x) = 3x^4 - 2x^3 + 5x - 7$ by $x - 2$.

Without performing the division, calculate the remainder when dividing the polynomial $P(x) = 3x^4 - 2x^3 + 5x - 7$ by $x - 2$.

Solve
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Calculate the numerical value of the polynomial $P(x) = -x^5 + 3x^3 - 2x^2 + x - 5$ for $x = -1$ and state the meaning of this value with respect to the polynomial…

Calculate the numerical value of the polynomial $P(x) = -x^5 + 3x^3 - 2x^2 + x - 5$ for $x = -1$ and state the meaning of this value with respect to the polynomial division of $P(x)$ by $x + 1$.

Solve
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Determine the remainder of the division of $P(x) = \frac{1}{2}x^3 - \frac{3}{2}x^2 + x - \frac{1}{2}$ by $x - 2$ by evaluating the polynomial.

Determine the remainder of the division of $P(x) = \frac{1}{2}x^3 - \frac{3}{2}x^2 + x - \frac{1}{2}$ by $x - 2$ by evaluating the polynomial.

Solve
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The position of an object moving in a straight line as a function of time $t$ (in seconds) is given by the polynomial $x(t) = -2t^3 + 8t^2 - 5t + 3$ (in meters). Use the…

The position of an object moving in a straight line as a function of time $t$ (in seconds) is given by the polynomial $x(t) = -2t^3 + 8t^2 - 5t + 3$ (in meters). Use the Remainder Theorem to calculate the position of the object at time $t = 3$ and explain the conceptual relationship between the value found and the polynomial division of $x(t)$ by $t - 3$.

Solve

The factor theorem: when the remainder is zero

If the remainder $P(a)$ turns out to be $0$, the division is exact: $(x-a)$ is a factor of $P(x)$ and $a$ is a root.

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Determine whether the polynomial $Q(x) = x^3 + 4x^2 - x - 4$ is divisible by $x + 4$ by applying the Remainder Theorem.

Determine whether the polynomial $Q(x) = x^3 + 4x^2 - x - 4$ is divisible by $x + 4$ by applying the Remainder Theorem.

Solve
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Check using the Remainder Theorem whether $x = 3$ is a root of the polynomial $P(x) = x^3 - 4x^2 + 5x - 6$.

Check using the Remainder Theorem whether $x = 3$ is a root of the polynomial $P(x) = x^3 - 4x^2 + 5x - 6$.

Solve

Finding a parameter with one remainder condition

When the polynomial has an unknown coefficient and the remainder is known, you substitute in the same way, and the condition becomes an equation for that unknown.

Setting up an equation for the parameter k from the remainder of dividing by x minus 1. P(x) = 2x³ − kx² + 3x + 1, remainder of ÷(x−1) = 4 P(1) = 2 − k + 3 + 1 = 6 − k 6 − k = 4 → k = 2
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Find the value of the parameter $k$ so that the remainder of dividing $P(x) = 2x^3 - kx^2 + 3x + 1$ by $x - 1$ is $4$.

Find the value of the parameter $k$ so that the remainder of dividing $P(x) = 2x^3 - kx^2 + 3x + 1$ by $x - 1$ is $4$.

Solve
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Find the value of $m$ knowing that the division of the polynomial $R(x) = x^4 + mx^3 - 2x^2 + 5x - 6$ by $x - 2$ is exact.

Find the value of $m$ knowing that the division of the polynomial $R(x) = x^4 + mx^3 - 2x^2 + 5x - 6$ by $x - 2$ is exact.

Solve
Practice★☆☆Not started
Knowing that the remainder of dividing $P(x) = x^3 - 2x^2 + ax + 5$ by $x + 2$ is $-7$, calculate the value of the coefficient $a$.

Knowing that the remainder of dividing $P(x) = x^3 - 2x^2 + ax + 5$ by $x + 2$ is $-7$, calculate the value of the coefficient $a$.

Solve
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Determine the value of $k$ so that the remainder of dividing the polynomial $P(x) = 2x^3 - kx^2 + x - k$ by $x - 2$ is identical to the remainder of dividing the…

Determine the value of $k$ so that the remainder of dividing the polynomial $P(x) = 2x^3 - kx^2 + x - k$ by $x - 2$ is identical to the remainder of dividing the polynomial $Q(x) = x^2 - kx + 3$ by $x + 1$.

Solve

Several conditions at once: systems of equations

With two parameters you need two conditions; each gives an equation, and together they form a system of linear equations, which you solve with the method you prefer (elimination, substitution, or equating).

Setting up a system of two equations from two conditions on the remainder of a polynomial. P(x) = x³ + mx² + nx − 10 P(2) = 0 (divisible by x−2) P(−2) = −24 (remainder when dividing by x+2) → system of two equations in m and n

Notice that a condition such as "the constant term is $-12$" or "it is divisible by $x$" also provides an equation: $P(0)=-12$ or $P(0)=0$. And if it is divisible by $x^2-4=(x-2)(x+2)$, then it is divisible by $x-2$ and by $x+2$: $P(2)=0$ and $P(-2)=0$.

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Find the values of the parameters $m$ and $n$ so that the polynomial $P(x) = x^3 + mx^2 + nx - 10$ is divisible by $x - 2$ and leaves a remainder of $-24$ when divided…

Find the values of the parameters $m$ and $n$ so that the polynomial $P(x) = x^3 + mx^2 + nx - 10$ is divisible by $x - 2$ and leaves a remainder of $-24$ when divided by $x + 2$.

Solve
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Calculate the values of $a$ and $b$ knowing that the polynomial $P(x) = ax^4 + bx^3 - x^2 + 2x - 4$ is simultaneously divisible by $(x - 1)$ and by $(x + 1)$.

Calculate the values of $a$ and $b$ knowing that the polynomial $P(x) = ax^4 + bx^3 - x^2 + 2x - 4$ is simultaneously divisible by $(x - 1)$ and by $(x + 1)$.

Solve
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A cubic polynomial $P(x)$ has leading coefficient $2$. Knowing that it is divisible by $(x - 1)$ and by $(x + 2)$, and that when divided by $(x - 2)$ it leaves a…

A cubic polynomial $P(x)$ has leading coefficient $2$. Knowing that it is divisible by $(x - 1)$ and by $(x + 2)$, and that when divided by $(x - 2)$ it leaves a remainder of $16$, determine the full expression of the polynomial $P(x)$ in expanded form.

Solve
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The heat capacity of an experimental material is approximated by the cubic polynomial $C(T) = aT^3 + bT^2 + cT + d$. It is known that $C(T)$ is divisible by $T$, that at…

The heat capacity of an experimental material is approximated by the cubic polynomial $C(T) = aT^3 + bT^2 + cT + d$. It is known that $C(T)$ is divisible by $T$, that at $T=1$ its value is $4$, at $T=-1$ its value is $0$, and at $T=2$ its value is $18$. Formulate the corresponding system of equations using the Remainder Theorem and find the algebraic expression for the polynomial $C(T)$.

Solve
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Determine the coefficients $a, b, c, d$ of the polynomial $P(x) = x^4 + ax^3 + bx^2 + cx + d$ knowing that it is divisible by the binomial $x^2 - 4$, that when divided…

Determine the coefficients $a, b, c, d$ of the polynomial $P(x) = x^4 + ax^3 + bx^2 + cx + d$ knowing that it is divisible by the binomial $x^2 - 4$, that when divided by $x - 1$ it leaves a remainder of $15$, and that its constant term is $-12$.

Solve

Applications: evaluating, proving, and modeling

The remainder theorem is also useful for evaluating models, checking roots, and solving for constants in real-world contexts.

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The production cost of $q$ thousand units of an electronic component follows the formula $C(q) = 10q^4 - 60q^3 + 100q^2 + 240$ (in thousands of pesos). Apply the…

The production cost of $q$ thousand units of an electronic component follows the formula $C(q) = 10q^4 - 60q^3 + 100q^2 + 240$ (in thousands of pesos). Apply the Remainder Theorem to determine the exact cost of manufacturing $4000$ units ($q=4$).

Solve
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The growth of a bacterial colony in a controlled environment is modelled by the polynomial $P(t) = t^4 - 7t^3 + 17t^2 - 17t + 6$, where $t$ is the time in days and…

The growth of a bacterial colony in a controlled environment is modelled by the polynomial $P(t) = t^4 - 7t^3 + 17t^2 - 17t + 6$, where $t$ is the time in days and $P(t)$ represents the deviation from the initial population. Prove, using the Factor Theorem, that on days $t=1$, $t=2$, and $t=3$ the population returns exactly to its initial level.

Solve
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The deflection of a beam at a point $x$ (in meters) is given by the polynomial $D(x) = kx^4 - 2x^3 + 3x^2$. Knowing that the deflection at $x = 2$ is $20$ mm, find the…

The deflection of a beam at a point $x$ (in meters) is given by the polynomial $D(x) = kx^4 - 2x^3 + 3x^2$. Knowing that the deflection at $x = 2$ is $20$ mm, find the value of the constant $k$ using the Remainder Theorem and determine what the deflection will be at coordinate $x = -1$.

Solve
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The profile of a drainage channel in an architectural project follows the equation $y = P(x) = -kx^3 + 2kx^2 - 5x + c$, where $y = 0$ represents ground level. It is…

The profile of a drainage channel in an architectural project follows the equation $y = P(x) = -kx^3 + 2kx^2 - 5x + c$, where $y = 0$ represents ground level. It is known that the profile crosses ground level at the points $x = 5$ and $x = -2$. Find the exact values of $k$ and $c$ using the Factor Theorem, and determine the elevation of the profile (height relative to the ground, negative if below it) at the point $x = 1$.

Solve
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Let $P(x) = x^3 + ax^2 + bx - 4$ be a polynomial. Knowing that it is divisible by $x - 2$ and that the sum of the remainders of dividing it by $x - 1$ and by $x + 1$ is…

Let $P(x) = x^3 + ax^2 + bx - 4$ be a polynomial. Knowing that it is divisible by $x - 2$ and that the sum of the remainders of dividing it by $x - 1$ and by $x + 1$ is equal to $-10$, set up the relevant system of equations and determine the coefficients $a$ and $b$.

Solve

Huge exponents

The remainder theorem does not distinguish between a degree-$3$ polynomial and a degree-$2024$ one: substituting costs just as little, as long as you group the powers well.

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In a cryptographic security system, a key is generated by evaluating a polynomial of very high degree. The key corresponds exactly to the remainder of dividing the…

In a cryptographic security system, a key is generated by evaluating a polynomial of very high degree. The key corresponds exactly to the remainder of dividing the polynomial $P(x) = x^{2024} - 3x^{2023} + 2x^5 - 7x + 4$ by the binomial $x - 3$. Calculate the numerical value of this security key, justifying your answer.

Solve
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Prove, applying the Remainder Theorem, that the polynomial $P(x) = x^n - a^n$ is always divisible by $x - a$ for any natural number $n$. Use this result to deduce, with…

Prove, applying the Remainder Theorem, that the polynomial $P(x) = x^n - a^n$ is always divisible by $x - a$ for any natural number $n$. Use this result to deduce, with reasoning, the remainder of the division of $x^5 - 32$ by $x - 2$.

Solve

The remainder when dividing by a degree-2 polynomial

If the divisor has degree $2$, the remainder can have degree $1$: $R(x)=mx+n$. If you know the two roots of the divisor, evaluating $P$ at each one gives two equations for $m$ and $n$.

Diagram for finding the remainder mx plus n by evaluating the polynomial at the two roots of the quadratic divisor. Q(x) = (x−1)(x−2), remainder R(x) = mx + n P(1) = m + n; P(2) = 2m + n two equations → m and n

If a third-degree polynomial leaves the same remainder $5$ in several divisions, then $P(x)-5$ vanishes at those roots: this lets you write it as a product of factors.

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Find the remainder when dividing the polynomial $P(x) = (x - 2)^{200} + (x - 1)^{100} - 5$ by the quadratic polynomial $Q(x) = (x - 1)(x - 2)$. Note: Remember that the…

Find the remainder when dividing the polynomial $P(x) = (x - 2)^{200} + (x - 1)^{100} - 5$ by the quadratic polynomial $Q(x) = (x - 1)(x - 2)$.

Note: Remember that the remainder when dividing by a polynomial of degree 2 is at most of the form $mx + n$. Apply the Remainder Theorem by strategically evaluating at the roots of the divisor.

Solve
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Find the expanded expression of a cubic polynomial $P(x)$ such that, when divided by $x+1$, $x-2$, and $x+3$, it leaves the same remainder equal to $5$ in all cases, and…

Find the expanded expression of a cubic polynomial $P(x)$ such that, when divided by $x+1$, $x-2$, and $x+3$, it leaves the same remainder equal to $5$ in all cases, and when divided by $x-1$ it leaves a remainder of $-7$.

Note: Think about the relationship between the auxiliary polynomial $P(x) - 5$ and the Factor Theorem.

Solve

Summary

Go back to the beginning: the remainder of dividing $x^{2024}-3x^{2023}+2x^5-7x+4$ by $x-3$ is $P(3)=469$.

See all the exercises in this section