Basic Concepts of Functions
A taxi in Mexico City charges a starting fee and then a set amount for each kilometer. If you know how many kilometers you are going to travel, you can know how much you will pay. That relationship between two quantities, where one depends on the other through a rule, is a function. By the end of this page you will be able to calculate the cost of a trip and tell how many kilometers you traveled from what you paid.
Function, domain, and variable
Example: at a bike rental shop, the total cost depends on the hours of use. The hours are chosen; the cost is calculated.
To evaluate a function, you substitute the value of $x$ and calculate. For example, if $f(x) = x^2 + 1$, then $f(3) = 3^2 + 1 = 10$.
Given the polynomial function $f(x) = 2x^2 - 5x + 1$, calculate the images for the values $x = -2$, $x = 0$ and $x = 3$.
Given the polynomial function $f(x) = 2x^2 - 5x + 1$, calculate the images for the values $x = -2$, $x = 0$ and $x = 3$.
A bicycle rental service charges a fixed fee of $60$ pesos to register for the service, plus $30$ pesos per hour of use. Write the algebraic expression of the function…
A bicycle rental service charges a fixed fee of $60$ pesos to register for the service, plus $30$ pesos per hour of use. Write the algebraic expression of the function that relates the total cost to the time of use, and identify which is the independent variable and which is the dependent variable.
A taxi company charges a base fare of $60$ pesos and $24$ pesos per kilometer traveled. a) Write the function relating the total price of the trip to the kilometers…
A taxi company charges a base fare of $60$ pesos and $24$ pesos per kilometer traveled.
- a) Write the function relating the total price of the trip to the kilometers traveled.
- b) What is the domain of this function taking into account the context of the problem?
- c) How much will a trip of $15$ km cost?
- d) If a customer paid $420$ pesos, how many kilometers did they travel?
Domain and range
A polynomial (sums, differences, and products of $x$) can be evaluated at any number, so $D = \mathbb{R}$. You also have to think about the context: in the taxi example, kilometers cannot be negative, so the domain is $[0, +\infty)$.
Reading domain and range from a graph
The domain is read on the $X$-axis (from where to where the graph extends); the range, on the $Y$-axis (which heights it reaches). A closed point ($\bullet$) belongs to the graph, so its endpoint is written with a bracket. An open point ($\circ$) does not belong, so its endpoint is written with a parenthesis.
Here the graph goes from $x = -3$ (included) to $x = 3$ (excluded): $D = [-3, 3)$. The heights go from the minimum $-1$ (included) up to $2$, which is not reached: range $= [-1, 2)$.
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Fraction with $x$ in the denominator: exclude the values that make it zero.
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Square root (even index): the radicand must be greater than or equal to zero.
From the graph of the function $h(x)$, knowing that the $y$-coordinate of the open point at $x = -2$ is $1.25$, state its domain and range in interval notation.
From the graph of the function $h(x)$, knowing that the $y$-coordinate of the open point at $x = -2$ is $1.25$, state its domain and range in interval notation.
Determine the domain of definition of the following functions: a) $f(x) = \frac{3}{x - 5}$ b) $g(x) = \sqrt{x + 2}$
Determine the domain of definition of the following functions:
- a) $f(x) = \frac{3}{x - 5}$
- b) $g(x) = \sqrt{x + 2}$
Find the domain of definition of the function $f(x) = \frac{\sqrt{x + 4}}{x - 2}$.
Find the domain of definition of the function $f(x) = \frac{\sqrt{x + 4}}{x - 2}$.
Determine analytically the domain of definition of the function $f(x) = \sqrt{\frac{x - 3}{x + 2}}$. Express the result as a union of intervals.
Determine analytically the domain of definition of the function $f(x) = \sqrt{\frac{x - 3}{x + 2}}$. Express the result as a union of intervals.
Intercepts with the axes
Calculate analytically the points of intersection with the x-axis and the y-axis of the function $f(x) = x^2 - 6x + 8$.
Calculate analytically the points of intersection with the x-axis and the y-axis of the function $f(x) = x^2 - 6x + 8$.
Symmetry: even and odd functions
Determine algebraically whether the following functions are even, odd, or have no symmetry: a) $f(x) = x^4 - 2x^2$ b) $g(x) = x^3 - 3x + 1$
Determine algebraically whether the following functions are even, odd, or have no symmetry:
- a) $f(x) = x^4 - 2x^2$
- b) $g(x) = x^3 - 3x + 1$
Given the rational function $f(x) = \frac{x^2}{x^2 - 4}$: a) Prove analytically whether it is an even function, an odd function, or has no symmetry. b) Calculate the…
Given the rational function $f(x) = \frac{x^2}{x^2 - 4}$:
- a) Prove analytically whether it is an even function, an odd function, or has no symmetry.
- b) Calculate the points of intersection with the coordinate axes.
- c) Find the equations of all its asymptotes (vertical and horizontal).
Average rate of change (AROC)
If the AROC is positive, $f(b) > f(a)$; if it is negative, $f(b) < f(a)$; if it is zero, $f(b) = f(a)$.
With speeds: if $d(t)$ is the distance, the AROC on $[t_1, t_2]$ is the average speed over that time interval.
Calculate the average rate of change of the function $f(x) = x^2 - 4$ over the intervals $[-1, 2]$ and $[2, 5]$.
Calculate the average rate of change of the function $f(x) = x^2 - 4$ over the intervals $[-1, 2]$ and $[2, 5]$.
The distance $d$ (in meters) covered by a free-falling object as a function of time $t$ (in seconds) is approximated by the function $d(t) = 5t^2$. Calculate the average…
The distance $d$ (in meters) covered by a free-falling object as a function of time $t$ (in seconds) is approximated by the function $d(t) = 5t^2$. Calculate the average speed of the object (which corresponds to the average rate of change) between the first and third seconds ($t \in [1, 3]$), and between the third and fifth seconds ($t \in [3, 5]$).
Calculate the average rate of change of the function $f(x) = \frac{1}{x}$ over the intervals $[1, 2]$ and $[2, 4]$. What does the sign of the results indicate about the…
Calculate the average rate of change of the function $f(x) = \frac{1}{x}$ over the intervals $[1, 2]$ and $[2, 4]$. What does the sign of the results indicate about the monotonicity of the function over those intervals?
Given the quadratic function $f(x) = x^2 + kx$, determine the value of the parameter $k$ given that the average rate of change of the function over the interval $[1, 3]$…
Given the quadratic function $f(x) = x^2 + kx$, determine the value of the parameter $k$ given that the average rate of change of the function over the interval $[1, 3]$ is equal to $8$.
The vertex of a parabola as an extreme point
For a quadratic function $f(x) = ax^2 + bx + c$, the highest or lowest point of the graph is at the vertex, at
$$x_v = -\frac{b}{2a}$$If $a < 0$ the parabola opens downward and the vertex is a maximum; if $a > 0$ it opens upward and it is a minimum. (It is the midpoint between the two $X$-intercepts, when they exist.) To find the maximum or minimum value, evaluate $f(x_v)$.
The temperature $T$ (in degrees Celsius) inside an industrial oven from when it is turned on until it is turned off is given by the function $T(t) = -t^2 + 8t + 20$,…
The temperature $T$ (in degrees Celsius) inside an industrial oven from when it is turned on until it is turned off is given by the function $T(t) = -t^2 + 8t + 20$, where $t$ is the elapsed time in hours. At what moment is the maximum temperature reached? What is the value of this maximum temperature?
A farmer has $100$ meters of fencing to enclose a rectangular vegetable patch using the straight wall of an existing building, so that he only needs to fence three of…
A farmer has $100$ meters of fencing to enclose a rectangular vegetable patch using the straight wall of an existing building, so that he only needs to fence three of the four sides of the rectangle.
- a) Find the expression of the function that gives the area of the patch as a function of the length $x$ of one of the sides perpendicular to the wall.
- b) Determine the valid domain of definition for $x$ in the context of this problem.
- c) What dimensions should the patch have so that its area is maximum?
Observe the following graph of a function $f(x)$ and determine its intervals of increase and decrease, as well as the exact coordinates of its local maximums and…
Observe the following graph of a function $f(x)$ and determine its intervals of increase and decrease, as well as the exact coordinates of its local maximums and minimums.
Analyse the following graph of the function $f(x)$ and determine: a) Its domain and range. b) The equations of its asymptotes. c) The intervals of increase and decrease.…
Analyse the following graph of the function $f(x)$ and determine:
- a) Its domain and range.
- b) The equations of its asymptotes.
- c) The intervals of increase and decrease.
- d) The coordinates of its local extrema (maximums and minimums).
Asymptotes
Determine analytically the equations of the vertical and horizontal asymptotes of the rational function $f(x) = \frac{2x + 1}{x - 3}$.
Determine analytically the equations of the vertical and horizontal asymptotes of the rational function $f(x) = \frac{2x + 1}{x - 3}$.
Given the rational function $f(x) = \frac{ax^2 + 3}{2x^2 - x}$, determine the value of the parameter $a$ so that the function has a horizontal asymptote at the line…
Given the rational function $f(x) = \frac{ax^2 + 3}{2x^2 - x}$, determine the value of the parameter $a$ so that the function has a horizontal asymptote at the line $y = 4$. With this value of $a$ calculated, also find the equations of the vertical asymptotes of the function.
Piecewise and periodic functions
Periodic functions. A function is periodic if its graph repeats every certain interval; that interval is the period. The height of a Ferris wheel cabin, for example, repeats on each turn. The maximum and minimum are read directly from the graph, and the period is the horizontal distance between two consecutive equal points.
Given the following graph of a piecewise-defined function, find the values of $f(-2)$, $f(1)$ and $f(4)$. Then, state at which points the function is discontinuous and…
Given the following graph of a piecewise-defined function, find the values of $f(-2)$, $f(1)$ and $f(4)$. Then, state at which points the function is discontinuous and the type of each discontinuity.
The accompanying graph represents the height in meters of a Ferris wheel capsule as a function of time in minutes. Observe the graph and answer: a) Is it a periodic…
The accompanying graph represents the height in meters of a Ferris wheel capsule as a function of time in minutes. Observe the graph and answer:
- a) Is it a periodic function? If so, state its period.
- b) What are the maximum and minimum heights reached by the capsule?
- c) At what height is the capsule exactly $10$ minutes after starting to rotate?
Find the values of the parameters $a$ and $b$ so that the following piecewise-defined function is continuous throughout its domain:…
Find the values of the parameters $a$ and $b$ so that the following piecewise-defined function is continuous throughout its domain:
$f(x) = \begin{cases} 2x + a & \text{if } x < 1 \\ x^2 - 3x + 5 & \text{if } 1 \le x < 4 \\ bx - 1 & \text{if } x \ge 4 \end{cases}$
A public parking lot has the following charges: it charges $40$ pesos for the first hour (or fraction of an hour), and $30$ pesos for each additional hour (or fraction).…
A public parking lot has the following charges: it charges $40$ pesos for the first hour (or fraction of an hour), and $30$ pesos for each additional hour (or fraction). The maximum cost per day is $240$ pesos. Write the analytical expression of the piecewise-defined function relating the price to be paid to the parking time $t$ (in hours) for the first $5$ hours, clearly indicating the intervals of definition for each section.
Back to the beginning: the cost of the taxi is $C(x) = 60 + 24x$, with domain $[0, +\infty)$. A $15$ km trip costs $60 + 360 = \$420$, and if you paid $\$420$ you solve $60 + 24x = 420$ and get $x = 15$ km.