Quadratic function

Quadratic function

A quadratic function, in mathematics, is a polynomial function of the form f(x)=ax^2+bx+c ,!, where a e 0 ,!. The graph of a quadratic function is a parabola whose major axis is parallel to the "y"-axis.

The expression ax^2+bx+c in the definition of a quadratic function is a polynomial of degree 2 or a 2nd degree polynomial, because the highest exponent of x is 2.

If the quadratic function is set equal to zero, then the result is a quadratic equation. The solutions to the equation are called the roots of the equation or the zeros of the function.

Origin of word

The adjective "quadratic" comes from the Latin word "quadratum" for square. A term like "x"2 is called a square in algebra because it is the area of a "square" with side "x".

In general, a prefix quadr(i)- indicates the number 4. Examples are quadrilateral and quadrant. "Quadratum" is the Latin word for square because a square has four sides.


The two roots of the quadratic equation 0=ax^2+bx+c,!, where a e 0 ,! are

x = frac{-b pm sqrt{b^2 - 4 a c{2 a}.

This formula is called the quadratic formula.

* Let Delta = b^2-4ac ,
* If Delta > 0,!, then there are two distinct roots since sqrt{Delta} is a positive real number.
* If Delta = 0,!, then the two roots are equal, since sqrt{Delta} is zero.
* If Delta < 0,!, then the two roots are complex conjugates, since sqrt{Delta} is imaginary.

By letting r_1 = frac{-b + sqrt{b^2 - 4 a c{2 a} and r_2 = frac{-b - sqrt{b^2 - 4 a c{2 a} or vice versa, one can factor a x^2 + b x + c ,! as a(x - r_1)(x - r_2),!.

Forms of a quadratic function

A quadratic function can be expressed in three formats:
* f(x) = a x^2 + b x + c ,! is called the general form or polynomial form,
* f(x) = a(x - r_1)(x - r_2),! is called the factored form, where r_1 and r_2 are the roots of the quadratic equation, it is used in logistic map
* f(x) = a(x - h)^2 + k ,! is called the standard form or vertex form.

To convert the general form to factored form, one needs only the quadratic formula to determine the two roots r_1 and r_2 . To convert the general form to standard form, one needs a process called completing the square. To convert the factored form (or standard form) to general form, one needs to multiply, expand and/or distribute the factors.


Regardless of the format, the graph of a quadratic function is a parabola (as shown above).
* If a > 0 ,!, the parabola opens upward.
* If a < 0 ,!, the parabola opens downward.

The coefficient "a" controls the speed of increase (or decrease) of the quadratic function from the vertex, bigger positive "a" makes the function increase faster and the graph appear more closed.

The coefficients "b" and "a" together control the axis of symmetry of the parabola (also the "x"-coordinate of the vertex).

The coefficient "b" alone is the declivity of the parabola as it crosses the y-axis.

The coefficient "c" controls the height of the parabola, more specifically, it is the point were the parabola crosses the "y"-axis.


The "x"-intercepts of the graph are the same as the roots of the quadratic function (see above).


The vertex of a parabola is the place where it turns, hence, it's also called the turning point. If the quadratic function is in standard form, the vertex is (h, k),!. By the method of completing the square, one can turn the general form: f(x) = a x^2 + b x + c ,! to

: f(x) = aleft(x + frac{b}{2a} ight)^2 - frac{b^2-4ac}{4 a} ,

so the vertex of the parabola in the general form will be

: left(-frac{b}{2a}, -frac{Delta}{4 a} ight).

If the quadratic function is in factored form f(x) = a(x - r_1)(x - r_2) ,!

the average of the two roots, i.e.,

: frac{r_1 + r_2}{2} ,!

is the "x"-coordinate of the vertex, and hence the vertex is

: left(frac{r_1 + r_2}{2}, f(frac{r_1 + r_2}{2}) ight).!

The vertex is also the maximum point if a < 0 ,! or the minimum point if a > 0 ,!.

The vertical line

: x=h=-frac{b}{2a}

that passes through the vertex is also the axis of symmetry of the parabola.

*Maximum and minimum points

:The maximum or minimum of the function is always obtained at the vertex, the following method is another derivation of the same fact using calculus, the advantage of this method is that it works for more general functions.

:Taking f(x) = ax^2 + bx + c ,! as sample quadratic equation, to find its maximum or minimum points (which depends on a ,!, if a > 0 ,!, it has a minimum point, if a < 0,!, it has a maximum point) we have to first, take its derivative:

::f(x)=ax^2+bx+c Leftrightarrow ,!f'(x)=2ax+b ,!

:Then, we find the roots of f'(x),!:

::2ax+b=0 Rightarrow ,! 2ax=-b Rightarrow,! x=-frac{b}{2a}

:So, -frac{b} {2a} is the x,! value of f(x),!. Now, to find the y,! value, we substitute x = -frac{b} {2a} on f(x),!:

::y=a left (-frac{b}{2a} ight)^2+b left (-frac{b}{2a} ight)+cRightarrow y= frac{ab^2}{4a^2} - frac{b^2}{2a} + c Rightarrow y= frac{b^2}{4a} - frac{b^2}{2a} + c Rightarrow::y= frac{b^2 - 2b^2 + 4ac}{4a} Rightarrow y= frac{-b^2+4ac}{4a} Rightarrow y= -frac{(b^2-4ac)}{4a} Rightarrow y= -frac{Delta}{4a}

:Thus, the maximum or minimum point coordinates are:

:: left (-frac {b}{2a}, -frac {Delta}{4a} ight).

The square root of a quadratic function

The square root of a quadratic function gives rise either to an ellipse or to a hyperbola.If a>0,! then the equation y = pm sqrt{a x^2 + b x + c} describes a hyperbola. The axis of the hyperbola is determined by the ordinate of the minimum point of the corresponding parabola y_p = a x^2 + b x + c ,!.
If the ordinate is negative, then the hyperbola's axis is horizontal. If the ordinate is positive, then the hyperbola's axis is vertical.
If a<0,! then the equation y = pm sqrt{a x^2 + b x + c} describes either an ellipse or nothing at all. If the ordinate of the maximum point of the corresponding parabola y_p = a x^2 + b x + c ,! is positive, then its square root describes an ellipse, but if the ordinate is negative then it describes an empty locus of points.

Bivariate quadratic function

A bivariate quadratic function is a second-degree polynomial of the form: f(x,y) = A x^2 + B y^2 + C x + D y + E x y + F ,!Such a function describes a quadratic surface. Setting f(x,y),! equal to zero describes the intersection of the surface with the plane z=0,!, which is a locus of points equivalent to a conic section.


If 4AB-E^2 <0 , the function has no maximum or minimum, its graph forms an hyperbolic paraboloid.

If 4AB-E^2 >0 , the function has a minimum if "A">0, and a maximum if "A"<0, its graph forms an elliptic paraboloid.

The minimum or maximum of a bivariate quadratic function is obtained at (x_m, y_m) , where:

:x_m = -frac{2BC-DE}{4AB-E^2}

:y_m = -frac{2AD-CE}{4AB-E^2}

If 4AB- E^2 =0 , and DE-2CB=2AD-CE e 0 , the function has no maximum or minimum, its graph forms a parabolic cylinder.

If 4AB- E^2 =0 , and DE-2CB=2AD-CE =0 , the function achieves the maximum/minimum at a line. Similarly, a minimum if "A">0 and a maximum if "A"<0, its graph forms a parabolic cylinder.

ee also

* Quadratic form
* Quadratic polynomial
* Matrix representation of conic sections
* Quadric
* Periodic points of complex quadratic mappings
* List of mathematical functions

External links


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • quadratic function — noun Any function whose value is the solution of a quadratic polynomial …   Wiktionary

  • Quadratic programming — (QP) is a special type of mathematical optimization problem. It is the problem of optimizing (minimizing or maximizing) a quadratic function of several variables subject to linear constraints on these variables.The quadratic programming problem… …   Wikipedia

  • Quadratic equation — This article is about quadratic equations and solutions. For more general information about quadratic functions, see Quadratic function. For more information about quadratic polynomials, see Quadratic polynomial. In mathematics, a quadratic… …   Wikipedia

  • Quadratic — In mathematics, the term quadratic describes something that pertains to squares, to the operation of squaring, to terms of the second degree, or equations or formulas that involve such terms. Quadratus is Latin for square . Mathematics Algebra… …   Wikipedia

  • Quadratic polynomial — In mathematics, a quadratic polynomial is a polynomial whose degree is 2. A quadratic polynomial with three terms is called a quadratic trinomial. Some examples of quadratic polynomials are ax 2 + bx + c , 2 x 2 − y 2, and xy + xz + yz… …   Wikipedia

  • quadratic programming — Variant of linear programming in which the objective function is quadratic rather than linear. In portfolio selection, we often minimize the variance of the portfolio (which is a quadratic function) subject to constraints on the mean return of… …   Financial and business terms

  • quadratic — adjective Date: 1668 involving terms of the second degree at most < quadratic function > < quadratic equations > • quadratic noun • quadratically adverb …   New Collegiate Dictionary

  • Quadratic probing — is a scheme in computer programming for resolving collisions in hash tables.Quadratic probing operates by taking the original hash value and adding successive values of an arbitrary quadratic polynomial to the starting value. This algorithm is… …   Wikipedia

  • Quadratic residue — In number theory, an integer q is called a quadratic residue modulo n if it is congruent to a perfect square modulo n; i.e., if there exists an integer x such that: Otherwise, q is called a quadratic nonresidue modulo n. Originally an abstract… …   Wikipedia

  • Quadratic sieve — The quadratic sieve algorithm (QS) is a modern integer factorization algorithm and, in practice, the second fastest method known (after the general number field sieve). It is still the fastest for integers under 100 decimal digits or so, and is… …   Wikipedia