Find Range of Quadratic Functions

Find the range of quadratic functions; examples and matched problems with their answers are located at the bottom of this page.

Graphical Analysis of Range of Quadratic Functions

The range of a function y = f(x) is the set of values y takes for all values of x within the domain of f.

The graph of any quadratic function, of the form f(x) = a x
2 + b x + c, which can be written in vertex form as follows
f(x) = a(x - h)2 + k where h = - b / 2a and k = f(h)

is either a parabola opening up, when a > 0, or a parabola opening down, when a < 0 (see graphs of several quadratic function below).

Therefore if a > 0, the graph of f has a minimum point and if a < 0, the graph of f has a maximum point. Both minima or maxima are the vertices of the parabolas with coordinates (k , k) where h = - b / 2a and k = f(h).

Examples of Quadratic Functions
Fig1. - Examples of Quadratic Functions with Minima and Maxima.

Example 1: Find the range of function f defined by

f(x) = -2 x2 + 4 x + 2

Solution to Example 1

  • The vertex of the graph of f is at the point ( h , k ) where
    h = - b / 2 a = - 4 / 2(-2) = 1 and k = f(1) = 4
  • The leading coefficient a = - 2 is negative and therefore the graph of f has a maximum at the point (1 , 4). The maximum value of f is 4. Hence the range of f is therefore given by the interval: (-∞ , 4 ] (see graph below to better understand)
    Graph of Quadratic Function with Maximum
    Fig2. - Graph of Quadratic Function with Maximum.

Matched Problem 1: Find the range of function f defined by

f(x) = -3 x2 - 6 x


Example 2: Find the range of function f defined by

f(x) = 2 x2 + 12 x + 16

Solution to Example 2

  • The coordinates h and k of the vertex of the graph of f are given by
    h = - b / 2a = - 12 / 2(2) = - 3 and k = f(-3) = - 2


  • The leading coefficient a = 2 is positive and therefore the graph of f has a minimum point at (h , k) = (-3 , -2). The range of f is given by the interval [-2 , +∞ ) (see graph of f below)
    Graph of Quadratic Function with Minimum
    Fig2. - Graph of Quadratic Function with Minimum.

Matched Problem 2: Find the range of function f defined by

f(x) = 2 x2 - 10 x + 19




Answers for Matched Problems

1)       (-∞ , 3 ]

2)       [ 6.5 , +∞ )

More
Find domain and range of functions, Find the range of functions, find the domain of a function and mathematics tutorials and problems.




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