In algebra, polynomiallong division is an algorithm for dividing a polynomial by another polynomial of the same or lower degree, a generalised version of the familiar arithmetic technique called long division. It can be done easily by hand, because it separates an otherwise complex division problem into smaller ones. Sometimes using a shorthand version called synthetic division is faster, with less writing and fewer calculations. Another abbreviated method is polynomial short division. Polynomial long division is an algorithm that implements the Euclidean division of polynomials, which starting from two polynomials A and B produces, if B is not zero, a quotientQ and a remainderR such that and either R = 0 or the degree of R is lower than the degree of B. These conditions uniquely define Q and R, which means that Q and R do not depend on the method used to compute them. The result R = 0 occurs if and only if the polynomial A has B as a factor. Thus long division is a means for testing whether one polynomial has another as a factor, and, if it does, for factoring it out. For example, if a rootr of A is known, it can be factored out by dividing A by.
Example
Polynomial long division
Find the quotient and the remainder of the division of the dividend, by the divisor. The dividend is first rewritten like this: The quotient and remainder can then be determined as follows: The polynomial above the bar is the quotient q, and the number left over is the remainder r. The long division algorithm for arithmetic is very similar to the above algorithm, in which the variable x is replaced by the specific number 10.
Polynomial short division
Blomqvist's method is an abbreviated version of the long division above. This pen-and-paper method uses the same algorithm as polynomial long division, but mental calculation is used to determine remainders. This requires less writing, and can therefore be a faster method once mastered. The division is at first written in a similar way as long multiplication with the dividend at the top, and the divisor below it. The quotient is to be written below the bar from left to right. Divide the first term of the dividend by the highest term of the divisor. Place the result below the bar. x3 has been divided leaving no remainder, and can therefore be marked as used with a backslash. The result x2 is then multiplied by the second term in the divisor -3 = -3x2. Determine the partial remainder by subtracting -2x2- = x2. Mark -2x2 as used and place the new remainder x2 above it. Divide the highest term of the remainder by the highest term of the divisor. Place the result below the bar. x2 has been divided leaving no remainder, and can therefore be marked as used. The result x is then multiplied by the second term in the divisor -3 = -3x. Determine the partial remainder by subtracting 0x- = 3x. Mark 0x as used and place the new remainder 3x above it. Divide the highest term of the remainder by the highest term of the divisor. Place the result below the bar. 3x has been divided leaving no remainder, and can therefore be marked as used. The result 3 is then multiplied by the second term in the divisor -3 = -9. Determine the partial remainder by subtracting -4- = 5. Mark -4 as used and place the new remainder 5 above it. The polynomial below the bar is the quotient q, and the number left over is the remainder r.
Pseudocode
The algorithm can be represented in pseudocode as follows, where +, −, and × represent polynomial arithmetic, and / represents simple division of two terms: function n / d is require d ≠ 0 q ← 0 r ← n // At each step n = d × q + r while r ≠ 0 and degree ≥ degree do t ← lead / lead // Divide the leading terms q ← q + t r ← r − t × d return Note that this works equally well when degree < degree; in that case the result is just the trivial. This algorithm describes exactly the above paper and pencil method: is written on the left of the ")"; is written, term after term, above the horizontal line, the last term being the value of ; the region under the horizontal line is used to compute and write down the successive values of.
For every pair of polynomials such that B ≠ 0, polynomial division provides a quotientQ and a remainderR such that and either R=0 or degree < degree. Moreover is the unique pair of polynomials having this property. The process of getting the uniquely defined polynomials Q and R from A and B is called Euclidean division. Polynomial long division is thus an algorithm for Euclidean division.
Applications
Factoring polynomials
Sometimes one or more roots of a polynomial are known, perhaps having been found using the rational root theorem. If one root r of a polynomial P of degree n is known then polynomial long division can be used to factor P into the form where Q is a polynomial of degree n − 1. Q is simply the quotient obtained from the division process; since r is known to be a root of P, it is known that the remainder must be zero. Likewise, if more than one root is known, a linear factor in one of them can be divided out to obtain Q, and then a linear term in another root, s, can be divided out of Q, etc. Alternatively, they can all be divided out at once: for example the linear factors and can be multiplied together to obtain the quadratic factor which can then be divided into the original polynomial P to obtain a quotient of degree In this way, sometimes all the roots of a polynomial of degree greater than four can be obtained, even though that is not always possible. For example, if the rational root theorem can be used to obtain a single root of a quintic polynomial, it can be factored out to obtain a quartic quotient; the explicit formula for the roots of a quartic polynomial can then be used to find the other four roots of the quintic.
Finding tangents to polynomial functions
Polynomial long division can be used to find the equation of the line that is tangent to the graph of the function defined by the polynomial P at a particular point If R is the remainder of the division of P by then the equation of the tangent line at to the graph of the function is regardless of whether or not r is a root of the polynomial.
