The Remainder Theorem and the Factor Theorem
Your teacher writes $P(x) = x^{500} - 2x^{499} + 4x^3 - 5$ on the board and asks: "What is the remainder when this is divided by $x - 2$?" Nobody is going to do that long division by hand. But there is a one-line trick that gets the answer in under a minute. By the end of this page, you'll have it.
The Remainder Theorem
When you divide a polynomial $P(x)$ by $x - a$, the Division Algorithm says
$$P(x) = (x - a)\,Q(x) + R$$where $Q(x)$ is the quotient and $R$ is the remainder. Because the divisor has degree 1, the remainder has degree 0, so $R$ is just a number. Now plug in $x = a$. The term $(a - a)\,Q(a)$ is $0$, so only $R$ is left.
Calculate, without performing the division, the remainder of dividing the polynomial $P(x) = 3x^4 - 2x^3 + 5x - 7$ by $x - 2$.
Calculate, without performing the division, the remainder of dividing the polynomial $P(x) = 3x^4 - 2x^3 + 5x - 7$ by $x - 2$.
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.
Calculate the numerical value of the polynomial $P(x) = -x^5 + 3x^3 - 2x^2 + x - 5$ for $x = -1$ and indicate what meaning this value has 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 indicate what meaning this value has with respect to the polynomial division of $P(x)$ by $x + 1$.
The Factor Theorem: when the remainder is zero
If $P(a) = 0$, the remainder is $0$, so the division is exact. That means $x - a$ divides $P(x)$ evenly. It is the Remainder Theorem in the special case $R = 0$.
Look at the graph of $Q(x) = x^3 + 4x^2 - x - 4$. The factor $x + 4$ corresponds to the zero $x = -4$, where the graph crosses the $x$-axis.
Determine if the polynomial $Q(x) = x^3 + 4x^2 - x - 4$ is divisible by $x + 4$ by applying the remainder theorem.
Determine if the polynomial $Q(x) = x^3 + 4x^2 - x - 4$ is divisible by $x + 4$ by applying the remainder theorem.
Check using the remainder theorem if $x = 3$ is a root of the polynomial $P(x) = x^3 - 4x^2 + 5x - 6$.
Check using the remainder theorem if $x = 3$ is a root of the polynomial $P(x) = x^3 - 4x^2 + 5x - 6$.
The growth of a colony of bacteria in a controlled medium is modeled by the polynomial $P(t) = t^4 - 7t^3 + 17t^2 - 17t + 6$, where $t$ is the time in days and $P(t)$…
The growth of a colony of bacteria in a controlled medium is modeled 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.
Finding one unknown coefficient
Sometimes a coefficient is missing, but you know the remainder. Substitute as usual. The condition becomes an equation in the unknown.
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"The division is exact" or "divisible by $x - a$" means $P(a) = 0$.
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"The remainder is $r$" means $P(a) = r$.
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 $4$.
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.
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 $a$.
Remainders in real models
Polynomials often model real situations: height over time, cost, distance. The Remainder Theorem says that the value of the model at $x = a$ is the same number as the remainder of dividing by $x - a$. So when a problem asks for "the value at $t = 3$", you can think of it as evaluating, which is the same as finding a remainder.
Here is the height in feet of a drone $t$ seconds after launch: $h(t) = -t^3 + 6t^2 + 2t + 5$. In the graph below, the horizontal axis shows $t$.
The position of a moving body 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 feet). Use the…
The position of a moving body 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 feet). Use the remainder theorem to calculate the position of the moving body at the instant $t = 3$ and explain the conceptual relationship between the value found and the polynomial division of $x(t)$ by $t - 3$.
The production cost of $q$ thousand units of an electronic component follows the law $C(q) = 0.5q^4 - 3q^3 + 5q^2 + 12$ (in thousands of dollars). Apply the remainder…
The production cost of $q$ thousand units of an electronic component follows the law $C(q) = 0.5q^4 - 3q^3 + 5q^2 + 12$ (in thousands of dollars). Apply the remainder theorem to determine the exact cost of manufacturing $4000$ units ($q=4$).
The deflection of a beam at a point $x$ (in feet) is given by the polynomial $D(x) = kx^4 - 2x^3 + 3x^2$. Knowing that the deflection at $x = 2$ is $20$ inches, find the…
The deflection of a beam at a point $x$ (in feet) is given by the polynomial $D(x) = kx^4 - 2x^3 + 3x^2$. Knowing that the deflection at $x = 2$ is $20$ inches, find the value of the constant $k$ relying on the remainder theorem and determine what the deflection will be at the coordinate $x = -1$.
Two unknowns, two conditions
With two unknown coefficients you need two conditions. Each one gives an equation, and together they form a system. You can solve it by substitution or elimination, just like in Integrated Math I.
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 $x + 2$.
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 $(x + 1)$.
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 $x + 1$.
Larger systems with several coefficients
The same plan works for harder setups. Some tips:
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A condition on the constant term or "divisible by $x$" means $P(0) = 0$ or gives that constant directly.
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"Divisible by $x^2 - 4$" means divisible by $(x - 2)(x + 2)$, so $P(2) = 0$ and $P(-2) = 0$.
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"The sum of two remainders is $-10$" means add the two expressions $P(1) + P(-1)$ and set them equal to $-10$.
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Label each equation, simplify it, and then use elimination.
Let the polynomial be $P(x) = x^3 + ax^2 + bx - 4$. Knowing that it is divisible by $x - 2$ and that the sum of the remainders of dividing it by $x - 1$ and by $x + 1$…
Let the polynomial be $P(x) = x^3 + ax^2 + bx - 4$. 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 $b$.
The heat capacity of an experimental material is approximated by the third-degree polynomial $C(T) = aT^3 + bT^2 + cT + d$. We know that $C(T)$ is divisible by $T$, that…
The heat capacity of an experimental material is approximated by the third-degree polynomial $C(T) = aT^3 + bT^2 + cT + d$. We know 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 analytical expression of the polynomial $C(T)$.
Building a polynomial from zeros and remainders
The Factor Theorem works backward too. If you know the zeros $r_1, r_2, r_3$ of a cubic, then
$$P(x) = k\,(x - r_1)(x - r_2)(x - r_3),$$where $k$ is the leading coefficient. If $k$ isn't given, use one more piece of information to find it.
A useful trick: if $P(x)$ leaves the same remainder $r$ when divided by $x - a$ and $x - b$, then $P(a) = r$ and $P(b) = r$. So the new polynomial $P(x) - r$ has zeros $a$ and $b$.
A third-degree polynomial $P(x)$ has a leading coefficient of $2$. Knowing that it is divisible by $(x - 1)$ and by $(x + 2)$, and that when divided by $(x - 2)$ it…
A third-degree polynomial $P(x)$ has a leading coefficient of $2$. Knowing that it is divisible by $(x - 1)$ and by $(x + 2)$, and that when divided by $(x - 2)$ it gives a remainder of $16$, determine the complete expression of the polynomial $P(x)$ in its expanded form.
Find the expanded expression of a third-degree polynomial $P(x)$ such that, when divided by $x+1$, $x-2$ and $x+3$, it leaves in all cases the same remainder equal to…
Find the expanded expression of a third-degree polynomial $P(x)$ such that, when divided by $x+1$, $x-2$ and $x+3$, it leaves in all cases the same remainder equal to $5$, and that 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.
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 $x = 1$.
Huge exponents are no problem
The Remainder Theorem doesn't care whether the degree is $3$ or $2024$. You still just substitute. Often the big powers cancel, and you can see it when you rewrite $a \cdot a^{n-1} = a^n$.
In a cryptographic security system, a key is generated by evaluating a polynomial of a 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 a 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.
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 logically…
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 logically deduce the remainder of the division of $x^5 - 32$ by $x - 2$.
Dividing by a degree-2 polynomial
When the divisor has degree $2$, the remainder can have degree $1$. It has the form $R(x) = mx + b$. If the divisor factors as $(x - r_1)(x - r_2)$, then
$$P(x) = (x - r_1)(x - r_2)\,Q(x) + mx + b.$$Plug in $x = r_1$ and $x = r_2$. The first term vanishes both times, so $P(r_1) = m r_1 + b$ and $P(r_2) = m r_2 + b$. That is a system of two equations in $m$ and $b$.
Find the remainder of dividing the polynomial $P(x) = (x - 2)^{200} + (x - 1)^{100} - 5$ by the second-degree polynomial $Q(x) = (x - 1)(x - 2)$. Note: Remember that the…
Find the remainder of dividing the polynomial $P(x) = (x - 2)^{200} + (x - 1)^{100} - 5$ by the second-degree polynomial $Q(x) = (x - 1)(x - 2)$.
Note: Remember that the remainder of 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.
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 $-12$.