Quadratic function

$x^2 - x - 2\!$

A quadratic function, in mathematics, is a polynomial function of the form

$f(x)=ax^2+bx+c,\quad a \ne 0.$

The graph of a quadratic function is a parabola whose axis of symmetry is parallel to the y-axis.

The expression ax² + bx + c in the definition of a quadratic function is a polynomial of degree 2 or second order, 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.

Origin of word

The adjective quadratic comes from the Latin word quadrātum (“square”). A term like x² 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.

Roots

Further information: Quadratic equation

The roots (zeros) of the quadratic function

$f(x) = ax^2+bx+c\,$

are the values of x for which f(x) = 0.

When the coefficients a, b, and c, are real or complex, the roots are

$x=\frac{-b \pm \sqrt{\Delta}}{2 a},$

where the discriminant is defined as

$\Delta = b^2 - 4 a c \, .$

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 standard form,
• $f(x) = a(x - x_1)(x - x_2)\,\!$ is called the factored form, where x₁ and x₂ are the roots of the quadratic equation, it is used in logistic map
• $f(x) = a(x - h)^2 + k \,\!$ is called the vertex form, where h and k are the x and y coordinates of the vertex, respectively.

To convert the standard form to factored form, one needs only the quadratic formula to determine the two roots x₁ and x₂. To convert the standard form to vertex form, one needs a process called completing the square. To convert the factored form (or vertex form) to standard form, one needs to multiply, expand and/or distribute the factors.

Graph

$f(x) = ax^2 |_{a=\{0.1,0.3,1,3\}} \!$

$f(x) = x^2 + bx |_{b=\{1,2,3,4\}} \!$

$f(x) = x^2 + bx |_{b=\{-1,-2,-3,-4\}} \!$

Regardless of the format, the graph of a quadratic function is a parabola (as shown above).

• If $a > 0 \,\!$ (or is a positive number), the parabola opens upward.
• If $a < 0 \,\!$ (or is a negative number), 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) which is at $x = -\frac{b}{2a}$.

The coefficient b alone is the declivity of the parabola as y-axis intercepts.

The coefficient c controls the height of the parabola, more specifically, it is the point where the parabola intercept the y-axis.

Vertex

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 vertex form, the vertex is $(h, k)\,\!$. By the method of completing the square, one can turn the general form

$f(x) = a x^2 + b x + c \,\!$

into

$f(x) = a\left(x + \frac{b}{2a}\right)^2 - \frac{b^2-4ac}{4 a} ,$

so the vertex of the parabola in the vertex form is

$\left(-\frac{b}{2a}, -\frac{\Delta}{4 a}\right).$

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

$\left(\frac{r_1 + r_2}{2}, f\left(\frac{r_1 + r_2}{2}\right)\right).\!$

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

Using calculus, the vertex point, being a maximum or minimum of the function, can be obtained by finding the roots of the derivative:

$f(x)=ax^2+bx+c \quad \Rightarrow \quad f'(x)=2ax+b \,\!,$

giving

$x=-\frac{b}{2a}$

with the corresponding function value

$f(x) = a \left (-\frac{b}{2a} \right)^2+b \left (-\frac{b}{2a} \right)+c = -\frac{(b^2-4ac)}{4a} = -\frac{\Delta}{4a} \,\!,$

so again the vertex point coordinates can be expressed as

$\left (-\frac {b}{2a}, -\frac {\Delta}{4a} \right).$

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.

Iteration

Given an f(x) = ax² + bx + c, one cannot always deduce the analytic form of f⁽ⁿ⁾(x), which means the nth iteration of f(x). (The superscript can be extended to negative number referring to the iteration of the inverse of f(x) if the inverse exists.) But there is one easier case, in which f(x) = a(x − x₀)² + x₀.

In such case, one has

$f(x)=a(x-x_0)^2+x_0=h^{(-1)}(g(h(x)))\,\!$,

where

$g(x)=ax^2\,\!$ and $h(x)=x-x_0\,\!$.

So by induction,

$f^{(n)}(x)=h^{(-1)}(g^{(n)}(h(x)))\,\!$

can be obtained, where g⁽ⁿ⁾(x) can be easily computed as

$g^{(n)}(x)=a^{2^{n}-1}x^{2^{n}}\,\!$.

Finally, we have

$f^{(n)}(x)=a^{2^n-1}(x-x_0)^{2^n}+x_0\,\!$,

in the case of f(x) = a(x − x₀)² + x₀.

See Topological conjugacy for more detail about such relationship between f and g. And see Complex quadratic polynomial for the chaotic behavior in the general iteration.

Bivariate (two variable) quadratic function

Further information: Quadric and Quadratic form

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.

Minimum/maximum

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 $DE-2CB=2AD-CE \ne 0 \,$ 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.

See also

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

External links

• Weisstein, Eric W., "Quadratic" from MathWorld.