Functions with Roots
At La Quebrada in Acapulco, a cliff diver takes $t=\sqrt{2h/9.8}$ seconds to fall from a height of $h$ meters. What would happen if you gave the formula a negative height? You would get the square root of a negative number, and that is not a real number. To find out which values a formula like this can be used with, you have to carefully solve the inequality for the radicand and give the exact domain as an interval. That is what you will learn here.
Domain with even index: solving the inequality for the radicand
If $g(x)$ is quadratic, the inequality does not always give a half-line.
Determine the domain of definition of the following function with an even-index root: $f(x) = \sqrt{3x - 12}$
Determine the domain of definition of the following function with an even-index root:
$f(x) = \sqrt{3x - 12}$
Find the domain of the quadratic function under a square root: $h(x) = \sqrt{16 - x^2}$
Find the domain of the quadratic function under a square root:
$h(x) = \sqrt{16 - x^2}$
Calculate the domain of the function containing an even-index root in the denominator: $f(x) = \frac{1}{\sqrt[4]{x^2 - 5x + 6}}$
Calculate the domain of the function containing an even-index root in the denominator:
$f(x) = \frac{1}{\sqrt[4]{x^2 - 5x + 6}}$
Domain with odd index: no restriction
Calculate the domain of definition of the function with an odd-index root: $g(x) = \sqrt[3]{2x^2 - 8x + 1}$
Calculate the domain of definition of the function with an odd-index root:
$g(x) = \sqrt[3]{2x^2 - 8x + 1}$
Fractions and sums of roots
Find the domain of the irrational function containing a rational expression: $f(x) = \sqrt{\frac{x - 2}{x + 5}}$
Find the domain of the irrational function containing a rational expression:
$f(x) = \sqrt{\frac{x - 2}{x + 5}}$
Find the domain of definition of the function formed by the sum of two roots: $f(x) = \sqrt{x + 4} + \sqrt{6 - 2x}$
Find the domain of definition of the function formed by the sum of two roots:
$f(x) = \sqrt{x + 4} + \sqrt{6 - 2x}$
Determine the domain of the irrational function whose radicand is an algebraic fraction with a quadratic numerator: $f(x) = \sqrt{\frac{x^2 - 9}{x - 1}}$
Determine the domain of the irrational function whose radicand is an algebraic fraction with a quadratic numerator:
$f(x) = \sqrt{\frac{x^2 - 9}{x - 1}}$
Nested roots and radicands that need factoring
When one root is inside another, the conditions are solved from the inside out.
Sometimes the inner root appears compared with an expression that can be negative. In that case it is best to split into cases.
Find the domain of definition of the nested function: $f(x) = \sqrt{2 - \sqrt{x - 3}}$
Find the domain of definition of the nested function:
$f(x) = \sqrt{2 - \sqrt{x - 3}}$
Determine the domain of the irrational function containing a rational expression where the numerator and denominator share roots:…
Determine the domain of the irrational function containing a rational expression where the numerator and denominator share roots:
$f(x) = \sqrt{\frac{x^3 - 3x^2 - 4x + 12}{x^2 + x - 2}}$
Solve the inequality associated with the domain of the function $f(x) = \sqrt{\sqrt{x + 7} - x - 1}$ and express it in interval notation.
Solve the inequality associated with the domain of the function $f(x) = \sqrt{\sqrt{x + 7} - x - 1}$ and express it in interval notation.
Domain that depends on a parameter
A parabola $y=x^2+kx+c$ opens upward. It is always $\geq0$ when it does not dip below the $x$-axis, that is, when its discriminant $\Delta=k^2-4c$ satisfies $\Delta\leq0$.
Determine the values of the parameter $k$ for which the domain of the function $f(x) = \sqrt{x^2 + kx + 9}$ is the entire set of real numbers $\mathbb{R}$.
Determine the values of the parameter $k$ for which the domain of the function $f(x) = \sqrt{x^2 + kx + 9}$ is the entire set of real numbers $\mathbb{R}$.
Shape of the graph, range, and translations
From the function $f(x) = \sqrt{x}$, write the algebraic expression for the function $g(x)$ whose graph is obtained by translating $f(x)$ three units to the left and two…
From the function $f(x) = \sqrt{x}$, write the algebraic expression for the function $g(x)$ whose graph is obtained by translating $f(x)$ three units to the left and two units upwards. What is the domain of $g(x)$?
Determine the domain and range of the function with a negative leading coefficient: $f(x) = -\sqrt{x - 1}$
Determine the domain and range of the function with a negative leading coefficient:
$f(x) = -\sqrt{x - 1}$
Observe the attached graph representing a function of the form $f(x) = a\sqrt{x - b} + c$. Determine the values of the parameters $a$, $b$ and $c$, knowing that the…
Observe the attached graph representing a function of the form $f(x) = a\sqrt{x - b} + c$. Determine the values of the parameters $a$, $b$ and $c$, knowing that the vertex of the semi-parabola is located at the point $(2, 1)$ and it passes through the point $(6, -3)$.
Intercepts with the axes and intersections
Find the axis intercepts of the function: $f(x) = \sqrt{x + 9} - 3$
Find the axis intercepts of the function:
$f(x) = \sqrt{x + 9} - 3$
Calculate the points of intersection between the irrational function $f(x) = \sqrt{2x + 5}$ and the linear function $g(x) = x + 1$.
Calculate the points of intersection between the irrational function $f(x) = \sqrt{2x + 5}$ and the linear function $g(x) = x + 1$.
Inverse function
To find the inverse function, swap $x$ and $y$ and solve for the new $y$. The domain of $f^{-1}$ is the range of $f$, and the range of $f^{-1}$ is the domain of $f$. If $f$ is not one-to-one (like $\sqrt{x^2-4x}$ over its whole domain), you must first restrict the domain to one branch.
Given the function $f(x) = 3 + \sqrt{2x - 4}$, determine its inverse function $f^{-1}(x)$ and state the domain and range of this inverse function.
Given the function $f(x) = 3 + \sqrt{2x - 4}$, determine its inverse function $f^{-1}(x)$ and state the domain and range of this inverse function.
Given the function $f(x) = \sqrt{x^2 - 4x}$, calculate its inverse function $f^{-1}(x)$ by restricting the original domain to the interval $(-\infty, 0]$. Clearly state…
Given the function $f(x) = \sqrt{x^2 - 4x}$, calculate its inverse function $f^{-1}(x)$ by restricting the original domain to the interval $(-\infty, 0]$. Clearly state the domain and range of the resulting inverse function.
Roots in geometry and physics
Whenever one quantity depends on the square of another and you need to solve for the second, a root appears: oscillation periods, fall times, radii from areas.
The relationship between the area $A$ of a circle and its radius $r$ is given by the formula $A = \pi r^2$. Express the radius in terms of the area, $r(A)$, and state…
The relationship between the area $A$ of a circle and its radius $r$ is given by the formula $A = \pi r^2$. Express the radius in terms of the area, $r(A)$, and state the valid domain of definition in this geometric context.
The period $T$ (in seconds) of a simple pendulum can be approximated by the function $T(L) = 2\pi\sqrt{\frac{L}{9.8}}$, where $L$ is the length of the pendulum in…
The period $T$ (in seconds) of a simple pendulum can be approximated by the function $T(L) = 2\pi\sqrt{\frac{L}{9.8}}$, where $L$ is the length of the pendulum in meters. Calculate the length required for the period to be exactly $2$ seconds and state the theoretical domain of the function $T(L)$ in this physical context.
The production cost $C(x)$, in pesos, of $x$ parts of an electronic component for satellites follows a model of diminishing returns given by…
The production cost $C(x)$, in pesos, of $x$ parts of an electronic component for satellites follows a model of diminishing returns given by $C(x) = 50\,000 + 2\,400\sqrt{x - 50}$. Determine the valid domain for the cost function and calculate how many parts were manufactured if the total cost amounted to $98\,000$ pesos.
Models with a restricted domain
In a model, the domain of the formula is combined with the domain of the context: you take the intersection of both conditions.
In the theory of special relativity, the Lorentz factor $\gamma$ depends on the velocity $v$ of an object according to the function…
In the theory of special relativity, the Lorentz factor $\gamma$ depends on the velocity $v$ of an object according to the function $\gamma(v) = \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}}$, where $c$ is the speed of light in a vacuum (approximately $3 \times 10^8$ m/s). Determine the domain of the function $\gamma(v)$ and interpret physically what happens to the function as $v$ approaches the boundaries of this domain.
A rectangle is inscribed in the region bounded by the x-axis and the upper semicircle of radius $R = 5$, whose equation is $y = \sqrt{25 - x^2}$. Express the perimetre…
A rectangle is inscribed in the region bounded by the x-axis and the upper semicircle of radius $R = 5$, whose equation is $y = \sqrt{25 - x^2}$. Express the perimetre of the rectangle $P(x)$ in terms of the coordinate $x$ of the vertex located in the first quadrant. Determine the valid domain of $P(x)$ in this geometric context and calculate its value for $x = 3$.
An engineering company estimates that the daily revenue $I(x)$ and costs $C(x)$, in thousands of pesos, when manufacturing $x$ metric tons of a composite material are…
An engineering company estimates that the daily revenue $I(x)$ and costs $C(x)$, in thousands of pesos, when manufacturing $x$ metric tons of a composite material are given by $I(x) = x\sqrt{40 - x}$ and $C(x) = 2x + 5$. Determine the domain of the profit function $B(x) = I(x) - C(x)$ in the context of the problem and calculate whether the company makes a profit or a loss when manufacturing $15$ metric tons.
Back to the beginning: in $t=\sqrt{2h/9.8}$ we must require $h\geq0$, so the domain is $[0,+\infty)$, and in the diver's context $h>0$.