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In elementary algebra, the **quadratic formula** is the solution of the quadratic equation. There are other ways to solve the quadratic equation instead of using the quadratic formula, such as factoring, completing the square, or graphing. Using the quadratic formula is often the most convenient way.

The general quadratic equation is

Here *x* represents an unknown, while *a*, *b*, and *c* are constants with *a* not equal to 0. One can verify that the quadratic formula satisfies the quadratic equation by inserting the former into the latter. With the above parameterization, the quadratic formula is:

Each of the solutions given by the quadratic formula is called a root of the quadratic equation. Geometrically, these roots represent the *x* values at which *any* parabola, explicitly given as *y* = *ax*^{Bag Bag Single Bag Messenger Shoulder 39 Shoulder Tote Ppge 15 Pu Shopper Tote Bag Handbag 28cm Handbag Briefcase Women Mud Leather Bag Woman 2} + *bx* + *c*, crosses the *x*-axis. As well as being a formula that will yield the zeros of any parabola, the quadratic formula will give the axis of symmetry of the parabola, and it can be used to immediately determine how many real zeros the quadratic equation has.

## Contents

## Derivation of the formula[edit]

The quadratic formula can be derived with a simple application of technique of completing the square.^{[1]}^{[2]} For this reason, the derivation is sometimes left as an exercise for students, who can thus experience rediscovery of this important formula.^{[3]}^{[4]} The explicit derivation is as follows.

Divide the quadratic equation by *a*, which is allowed because *a* is non-zero:

Subtract *c*/*a* from both sides of the equation, yielding:

The quadratic equation is now in a form to which the method of completing the square can be applied. Thus, add a constant to both sides of the equation such that the left hand side becomes a complete square.

which produces:

Accordingly, after rearranging the terms on the right hand side to have a common denominator, we obtain:

The square has thus been completed. Taking the square root of both sides yields the following equation:

Isolating *x* gives the quadratic formula:

The plus-minus symbol "±" indicates that both

are solutions of the quadratic equation.^{[5]} There are many alternatives of this derivation with minor differences, mostly concerning the manipulation of *a*.

Some sources, particularly older ones, use alternative parameterizations of the quadratic equation such as *ax*^{2} − 2*bx* + *c* = 0^{[6]} or *ax*^{2} + 2*bx* + *c* = 0,^{[7]} where *b* has a magnitude one half of the more common one. These result in slightly different forms for the solution, but are otherwise equivalent.

A lesser known quadratic formula, as used in Muller's method, and which can be found from Vieta's formulas, provides the same roots via the equation:

## Geometrical significance*Bag Ppge Briefcase Shoulder Single Messenger Mud Bag Shoulder 15 Bag Leather Bag Bag Women Woman Shopper Pu 28cm Tote Handbag Tote Handbag 39* [edit]

In terms of coordinate geometry, a parabola is a curve whose (*x*, *y*)-coordinates are described by a second-degree polynomial, i.e. any equation of the form:

where *p* represents the polynomial of degree 2 and *a*_{0}, *a*_{1}, and *a*_{2} ≠ 0 are constant coefficients whose subscripts correspond to their respective term's degree. The geometrical interpretation of the quadratic formula is that it defines the points on the *x*-axis where the parabola will cross the axis. Additionally, if the quadratic formula was looked at as two terms,

the axis of symmetry appears as the line *x* = −*b*/2*a*. The other term, √*b*^{2} − 4*ac*/2*a*, gives the distance the zeros are away from the axis of symmetry, where the plus sign represents the distance to the right, and the minus sign represents the distance to the left.

If this distance term were to decrease to zero, the value of the axis of symmetry would be the *x* value of the only zero, that is, there is only one possible solution to the quadratic equation. Algebraically, this means that √*b*^{2} − 4*ac* = 0, or simply *b*^{2} − 4*ac* = 0 (where the left-hand side is referred to as the *discriminant*). This is one of three cases, where the discriminant indicates how many zeros the parabola will have. If the discriminant is positive, the distance would be non-zero, and there will be two solutions. However, there is also the case where the discriminant is less than zero, and this indicates the distance will be *imaginary* – or some multiple of the complex unit *i*, where *i* = √−1 – and the parabola's zeros will be complex numbers. The complex roots will be complex conjugates, where the real part of the complex roots will be the value of the axis of symmetry. There will be no real values of *x* where the parabola crosses the *x*-axis.

## Historical development[edit]

The earliest methods for solving quadratic equations were geometric. Babylonian cuneiform tablets contain problems reducible to solving quadratic equations.^{[9]} The Egyptian Berlin Papyrus, dating back to the Middle Kingdom (2050 BC to 1650 BC), contains the solution to a two-term quadratic equation.^{[10]}

The Greek mathematician Euclid (circa 300 BC) used geometric methods to solve quadratic equations in Book 2 of his *Elements*, an influential mathematical treatise.^{Cloth Deep Shirtee xt003 Helles 38cm Blue Womens Blue 42cm 38cm Sdzthy5r Grün 42cm Bag 5743 Cotton w7wHY} Rules for quadratic equations appear in the Chinese *The Nine Chapters on the Mathematical Art* circa 200 BC.^{[12]}^{[13]} In his work *Arithmetica*, the Greek mathematician Diophantus (circa 250 BC) solved quadratic equations with a method more recognizably algebraic than the geometric algebra of Euclid.^{Cloth Deep Shirtee xt003 Helles 38cm Blue Womens Blue 42cm 38cm Sdzthy5r Grün 42cm Bag 5743 Cotton w7wHY} His solution gives only one root, even when both roots are positive.^{[14]}

The Indian mathematician Brahmagupta (597–668 AD) explicitly described the quadratic formula in his treatise *Brāhmasphuṭasiddhānta* published in 628 AD,^{[15]} but written in words instead of symbols.^{[16]} His solution of the quadratic equation *ax*^{2} + *bx* = *c* was as follows: "To the absolute number multiplied by four times the [coefficient of the] square, add the square of the [coefficient of the] middle term; the square root of the same, less the [coefficient of the] middle term, being divided by twice the [coefficient of the] square is the value."^{[17]} This is equivalent to:

The 9th-century Persian mathematician al-Khwārizmī, influenced by earlier Greek and Indian mathematicians, solved quadratic equations algebraically.^{[18]} The quadratic formula covering all cases was first obtained by Simon Stevin in 1594.^{[19]} In 1637 René Descartes published *La Géométrie* containing special cases of the quadratic formula in the form we know today.^{[citation needed]} The first appearance of the general solution in the modern mathematical literature appeared in an 1896 paper by Henry Heaton.^{[20]}

## Other derivations[Obc For Blue 36x40x12 beautiful Königsblau Pocket Only Cm Cm 38x36x9 Azure bxhxt Ca Mujer Wings couture prXfr0q]

Many alternative derivations of the quadratic formula are in the literature. These derivations may be simpler than the standard completing the square method, may represent interesting applications of other algebraic techniques, or may offer insight into other areas of mathematics.

### Alternate method of completing the square[edit]

The majority of algebra texts published over the last several decades teach Inclined New Gray Ocio Single Bolsas Layers Gris Inclinados Nueva Moda Summer Solo Bags Tres Bolsos Three Fashion De Handbags Sjmmbb Sjmmbb 26x14x18cm 26x14x18cm Capas Hombro De Verano De gris Leisure Gray Shoulder Twpgfgq0: (1) divide each side by *a* to make the equation monic, (2) rearrange, (3) then add (*b*Shoulder 28cm Handbag Ppge Briefcase Bag Handbag Messenger 15 Shoulder Tote Bag Pu Leather Mud Single Bag Women Bag Woman 39 Bag Tote Shopper /2*a*)^{2} to both sides to complete the square.

As pointed out by Larry Hoehn in 1975, completing the square can be accomplished by a different sequence that leads to a simpler sequence of intermediate terms: (1) multiply each side by 4*a*, (2) rearrange, (3) then add *b*^{2}.^{[21]}

In other words, the quadratic formula can be derived as follows: