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In elementary algebra, a quadratic equation (from the Latin quadratus for "square") is any equation having the form
where x represents an unknown, and a, b, and c are constants with a not equal to 0. If a = 0, then the equation is linear, not quadratic. The constants a, b, and c are called, respectively, the quadratic coefficient, the linear coefficient and the constant or free term.
Because the quadratic equation involves only one unknown, it is called "univariate". The quadratic equation only contains powers of x that are nonnegative integers, and therefore it is a polynomial equation, and in particular it is a second degree polynomial equation since the greatest power is two.
Quadratic equations can be solved by a process known in American English as factoring and in other varieties of English as factorising, by completing the square, by using the quadratic formula, or by graphing. Solutions to problems equivalent to the quadratic equation were known as early as 2000 BC.
A quadratic equation with real or complex coefficients has two solutions, called roots. These two solutions may or may not be distinct, and they may or may not be real.
It may be possible to express a quadratic equation ax^{2} + bx + c = 0 as a product (px + q)(rx + s) = 0. In some cases, it is possible, by simple inspection, to determine values of p, q, r, and s that make the two forms equivalent to one another. If the quadratic equation is written in the second form, then the "Zero Factor Property" states that the quadratic equation is satisfied if px + q = 0 or rx + s = 0. Solving these two linear equations provides the roots of the quadratic.
For most students, factoring by inspection is the first method of solving quadratic equations to which they are exposed.^{[1]}^{:202–207} If one is given a quadratic equation in the form x^{2} + px + q = 0, one would seek to find two numbers that add up to p and whose product is q ("Vieta's Rule"). The more general case where a does not equal 1 can require a considerable effort in trial and error guessandcheck, assuming that it can be factored at all by inspection.
Except for special cases such as where b = 0 or c = 0, factoring by inspection only works for quadratic equations that have rational roots. This means that the great majority of quadratic equations that arise in practical applications cannot be solved by factoring by inspection.^{[1]}^{:207}
The process of completing the square makes use of the algebraic identity
which represents a welldefined algorithm that can be used to solve any quadratic equation.^{[1]}^{:207} Starting with a quadratic equation in standard form, ax^{2} + bx + c = 0
We illustrate use of this algorithm by solving 2x^{2} + 4x − 4 = 0
The plusminus symbol "±" indicates that both x = −1 + √3 and x = −1 − √3 are solutions of the quadratic equation.^{[2]}
Completing the square can be used to derive a general formula for solving quadratic equations, called the quadratic formula.^{[3]} The mathematical proof will now be briefly summarized.^{[4]} It can easily be seen, by polynomial expansion, that the following equation is equivalent to the quadratic equation:
Taking the square root of both sides, and isolating x, gives:
A number of alternative derivations can be found in the literature which either (a) are simpler than the standard completing the square method, (b) represent interesting applications of other frequently used techniques in algebra, or (c) offer insight into other areas of mathematics.
It is sometimes convenient to reduce a quadratic equation to an equation involving two instead of three constant coefficients. This is done by simply dividing both sides by a, which is possible because a is nonzero. This produces the reduced quadratic equation:^{[7]}
Here p = b/a and q = c/a are the only coefficients in the reduced equation, which is also called a monic equation.
It follows from the quadratic formula that the solution to the reduced quadratic equation is
In the quadratic formula, the expression underneath the square root sign is called the discriminant of the quadratic equation, and is often represented using an upper case D or an upper case Greek delta:^{[8]}
A quadratic equation with real coefficients can have either one or two distinct real roots, or two distinct complex roots. In this case the discriminant determines the number and nature of the roots. There are three cases:
Thus the roots are distinct if and only if the discriminant is nonzero, and the roots are real if and only if the discriminant is nonnegative.
The function f(x) = ax^{2} + bx + c is the quadratic function.^{[10]} The graph of any quadratic function has the same general shape, which is called a parabola. The location and size of the parabola, and how it opens, depends on the values of a, b, and c. As shown in Figure 1, if a > 0, the parabola has a minimum point and opens upward. If a < 0, the parabola has a maximum point and opens downward. The extreme point of the parabola, whether minimum or maximum, corresponds to its vertex. The xcoordinate of the vertex will be located at , and the ycoordinate of the vertex may be found by substituting this xvalue into the function. The yintercept is located at the point (0, c).
The solutions of the quadratic equation ax^{2} + bx + c = 0 correspond to the roots of the function f(x) = ax^{2} + bx + c, since they are the values of x for which f(x) = 0. As shown in Figure 2, if a, b, and c are real numbers and the domain of f is the set of real numbers, then the roots of f are exactly the xcoordinates of the points where the graph touches the xaxis. As shown in Figure 3, if the discriminant is positive, the graph touches the xaxis at two points; if zero, the graph touches at one point; and if negative, the graph does not touch the xaxis.
The term
is a factor of the polynomial
if and only if r is a root of the quadratic equation
It follows from the quadratic formula that
In the special case b^{2} = 4ac where the quadratic has only one distinct root (i.e. the discriminant is zero), the quadratic polynomial can be factored as
For most of the 20th century, graphing was rarely mentioned as a method for solving quadratic equations in high school or college algebra texts. Students learned to solve quadratic equations by factoring, completing the square, and applying the quadratic formula. Recently, graphing calculators have become common in schools and graphical methods have started to appear in textbooks, but they are generally not highly emphasized.^{[11]}
Being able to use a graphing calculator to solve a quadratic equation requires the ability to produce a graph of y = f(x), the ability to scale the graph appropriately to the dimensions of the graphing surface, and the recognition that when f(x) = 0, x is a solution to the equation. The skills required to solve a quadratic equation on a calculator are in fact applicable to finding the real roots of any arbitrary function.
Since an arbitrary function may cross the xaxis at multiple points, graphing calculators generally require one to identify the desired root by positioning a cursor at a "guessed" value for the root. (Some graphing calculators require bracketing the root on both sides of the zero.) The calculator then proceeds, by an iterative algorithm, to refine the estimated position of the root to the limit of calculator accuracy.
Although the quadratic formula provides what in principle should be an exact solution, it does not, from a numerical analysis standpoint, provide a completely stable method for evaluating the roots of a quadratic equation. If the two roots of the quadratic equation vary greatly in absolute magnitude, b will be very close in magnitude to , and the subtraction of two nearly equal numbers will cause loss of significance or catastrophic cancellation. A second form of cancellation can occur between the terms b^{2} and −4ac of the discriminant, which can lead to loss of up to half of correct significant figures.^{[5]}^{[12]}
Babylonian mathematicians, as early as 2000 BC (displayed on Old Babylonian clay tablets) could solve problems relating the areas and sides of rectangles. In modern notation, the problems typically involved solving a pair of simultaneous equations of the form:
which are equivalent to the equation:^{[13]}^{:86}
The steps given by Babylonian scribes for solving the above rectangle problem were as follows:
Note that step (5) is essentially equivalent to calculating
There is evidence dating this algorithm as far back as the Ur III dynasty.^{[14]}
In the Sulba Sutras in ancient India circa 8th century BC quadratic equations of the form ax^{2} = c and ax^{2} + bx = c were explored using geometric methods. Babylonian mathematicians from circa 400 BC and Chinese mathematicians from circa 200 BC used geometric methods of dissection to solve quadratic equations with positive roots, but do not appear to have had a general formula.^{[15]}^{[16]}
Euclid, the Greek mathematician, produced a more abstract geometrical method around 300 BC. Pythagoras and Euclid used a strictly geometric approach, and found a general procedure to solve the quadratic equation. In his work Arithmetica, the Greek mathematician Diophantus solved the quadratic equation, but giving only one root, even when both roots were positive.^{[17]}
In 628 AD, Brahmagupta, an Indian mathematician, gave the first explicit (although still not completely general) solution of the quadratic equation ax^{2} + bx = c as follows:
This is equivalent to:
The Bakhshali Manuscript written in India in the 7th century AD contained an algebraic formula for solving quadratic equations, as well as quadratic indeterminate equations (originally of type ax/c = y
Muhammad ibn Musa alKhwarizmi (Persia, 9th century), inspired by Brahmagupta, developed a set of formulas that worked for positive solutions. AlKhwarizmi goes further in providing a full solution to the general quadratic equation, accepting one or two numerical answers for every quadratic equation, while providing geometric proofs in the process.^{[18]} He also described the method of completing the square and recognized that the discriminant must be positive,^{[18]}^{[19]}^{:230} which was proven by his contemporary 'Abd alHamīd ibn Turk (Central Asia, 9th century) who gave geometric figures to prove that if the discriminant is negative, a quadratic equation has no solution.^{[19]}^{:234} While alKhwarizmi himself did not accept negative solutions, later Islamic mathematicians that succeeded him accepted negative solutions,^{[18]}^{:191} as well as irrational numbers as solutions.^{[20]} Abū Kāmil Shujā ibn Aslam (Egypt, 10th century) in particular was the first to accept irrational numbers (often in the form of a square root, cube root or fourth root) as solutions to quadratic equations or as coefficients in an equation.^{[21]}
The Indian mathematician Sridhara, who flourished in the 9th and 10th centuries AC provided the modern solution of the quadratic equation.
The Jewish mathematician Abraham bar Hiyya HaNasi (12th century, Spain) authored the first European book to include the full solution to the general quadratic equation.^{[22]} His solution was largely based on AlKhwarizmi's work.^{[18]} The writing of the Chinese mathematician Yang Hui (1238–1298 AD) is the first known one in which quadratic equations with negative coefficients of 'x' appear, although he attributes this to the earlier Liu Yi.
By 1545 Gerolamo Cardano compiled the works related to the quadratic equations. The quadratic formula covering all cases was first obtained by Simon Stevin in 1594.^{[23]} In 1637 René Descartes published La Géométrie containing the quadratic formula in the form we know today. The first appearance of the general solution in the modern mathematical literature appeared in an 1896 paper by Henry Heaton.^{[24]}
Vieta's formulas give a simple relation between the roots of a polynomial and its coefficients. In the case of the quadratic polynomial, they take the following form:
and
These results follow immediately from the relation:
which can be compared term by term with
The first formula above yields a convenient expression when graphing a quadratic function. Since the graph is symmetric with respect to a vertical line through the vertex, when there are two real roots the vertex's xcoordinate is located at the average of the roots (or intercepts). Thus the xcoordinate of the vertex is given by the expression
The ycoordinate can be obtained by substituting the above result into the given quadratic equation, giving
As a practical matter, Vieta's formulas provide a useful method for finding the roots of a quadratic in the case where one root is much smaller than the other. If  x _{2} <<  x _{1}, then x _{1} + x _{2} ≈ x _{1}, and we have the estimate:
The second Vieta's formula then provides:
These formulas are much easier to evaluate than the quadratic formula under the condition of one large and one small root, because the quadratic formula evaluates the small root as the difference of two very nearly equal numbers (the case of large b), which causes roundoff error in a numerical evaluation. Figure 5 shows the difference between (i) a direct evaluation using the quadratic formula (accurate when the roots are near each other in value) and (ii) an evaluation based upon the above approximation of Vieta's formulas (accurate when the roots are widely spaced). As the linear coefficient b increases, initially the quadratic formula is accurate, and the approximate formula improves in accuracy, leading to a smaller difference between the methods as b increases. However, at some point the quadratic formula begins to lose accuracy because of round off error, while the approximate method continues to improve. Consequently the difference between the methods begins to increase as the quadratic formula becomes worse and worse.
This situation arises commonly in amplifier design, where widely separated roots are desired to ensure a stable operation (see step response).
In the days before calculators, people would use mathematical tables—lists of numbers showing the results of calculation with varying arguments—to simplify and speed up computation. Tables of logarithms and trigonometric functions were common in math and science textbooks. Specialized tables were published for applications such as astronomy, celestial navigation and statistics. Methods of numerical approximation existed, called prosthaphaeresis, that offered shortcuts around timeconsuming operations such as multiplication and taking powers and roots.^{[11]} Astronomers, especially, were concerned with methods that could speed up the long series of computations involved in celestial mechanics calculations.
It is within this context that we may understand the development of means of solving quadratic equations by the aid of trigonometric substitution. Consider the following alternate form of the quadratic equation,
[1]
where the sign of the ± symbol is chosen so that a and c may both be positive. By substituting
[2]
and then multiplying through by cos^{2}θ, we obtain
[3]
Introducing functions of 2θ and rearranging, we obtain
[4]
[5]
where the subscripts n and p correspond, respectively, to the use of a negative or positive sign in equation [1]. Substituting the two values of θ_{n} or θ_{p} found from equations [4] or [5] into [2] gives the required roots of [1]. Complex roots occur in the solution based on equation [5] if the absolute value of sin 2θ_{p} exceeds unity. The amount of effort involved in solving quadratic equations using this mixed trigonometric and logarithmic table lookup strategy was twothirds the effort using logarithmic tables alone.^{[25]} Calculating complex roots would require using a different trigonometric form.^{[26]}
The quadratic equation may be solved geometrically in a number of ways. One way is via Lill's method. The three coefficients a, b, c are drawn with right angles between them as in SA, AB, and BC in Figure 6. A circle is drawn with the start and end point SC as a diameter. If this cuts the middle line AB of the three then the equation has a solution, and the solutions are given by negative of the distance along this line from A divided by the first coefficient a or SA. If a is 1 the coefficients may be read off directly. Thus the solutions in the diagram are −AX1/SA and −AX2/SA.^{[27]}
The formula and its derivation remain correct if the coefficients a, b and c are complex numbers, or more generally members of any field whose characteristic is not 2. (In a field of characteristic 2, the element 2a is zero and it is impossible to divide by it.)
The symbol
in the formula should be understood as "either of the two elements whose square is b^{2} − 4ac, if such elements exist". In some fields, some elements have no square roots and some have two; only zero has just one square root, except in fields of characteristic 2. Note that even if a field does not contain a square root of some number, there is always a quadratic extension field which does, so the quadratic formula will always make sense as a formula in that extension field.
In a field of characteristic 2, the quadratic formula, which relies on 2 being a unit, does not hold. Consider the monic quadratic polynomial
over a field of characteristic 2. If b = 0, then the solution reduces to extracting a square root, so the solution is
and note that there is only one root since
In summary,
See quadratic residue for more information about extracting square roots in finite fields.
In the case that b ≠ 0, there are two distinct roots, but if the polynomial is irreducible, they cannot be expressed in terms of square roots of numbers in the coefficient field. Instead, define the 2root R(c) of c to be a root of the polynomial x^{2} + x + c, an element of the splitting field of that polynomial. One verifies that R(c) + 1 is also a root. In terms of the 2root operation, the two roots of the (nonmonic) quadratic ax^{2} + bx + c are
and
For example, let a denote a multiplicative generator of the group of units of F_{4}, the Galois field of order four (thus a and a + 1 are roots of x^{2} + x + 1 over F_{4}. Because (a + 1)^{2} = a, a + 1 is the unique solution of the quadratic equation x^{2} + a = 0. On the other hand, the polynomial x^{2} + ax + 1 is irreducible over F_{4}, but it splits over F_{16}, where it has the two roots ab and ab + a, where b is a root of x^{2} + x + a in F_{16}.
This is a special case of ArtinSchreier theory.
