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Integrals · Definite integral

Applications of the Definite Integral

Before you start, make sure you know: Tangent line to a curve (High School Year 2 (semesters 3 and 4))
Before you start, make sure you know: Quadratic equation (High School Year 2 (semesters 3 and 4))

The water tank on the roof of your house fills at a rate that changes during the morning. How do you know how many liters came in between 6:00 and 11:00 without measuring them one by one? You already know the definite integral and the Fundamental Theorem of Calculus (Barrow's rule). Here you use them for what they are really asked for: areas, accumulations, and functions defined by an integral.

Area under a curve with Barrow's rule

The procedure is always the same. First find the limits of integration (if they aren't given, they come from the intersection points). Then find an antiderivative $G$ and apply Barrow's rule: $\int_a^b f(x)\,dx = G(b)-G(a)$.

If the region lies below the $x$-axis, the integral comes out negative and the area is its absolute value.

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Calculate the value of the following definite integral applying Barrow's Rule: $$\int_{1}^{2} (3x^2 - 4x + 5) \, dx$$ <br>

Calculate the value of the following definite integral applying Barrow's Rule:

$$\int_{1}^{2} (3x^2 - 4x + 5) \, dx$$


1 2 x 4 9 y 0 f(x) = 3x² - 4x + 5 ? Area = ?
Solve
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A graphic designer is drawing the outline of a logo that follows the curve $f(x) = e^x$. Calculate the area enclosed under that curve, the $OX$ axis and the vertical…

A graphic designer is drawing the outline of a logo that follows the curve $f(x) = e^x$. Calculate the area enclosed under that curve, the $OX$ axis and the vertical lines $x = 0$ and $x = \ln(3)$.

Solve
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Calculate the area of the plane region bounded by the parabola $f(x) = x^2 - 4x + 3$ and the x-axis ($OX$). Note: Remember to find the intersection points of the…

Calculate the area of the plane region bounded by the parabola $f(x) = x^2 - 4x + 3$ and the x-axis ($OX$). Note: Remember to find the intersection points of the function with the axis first.


A 0 1 2 3 4 1 2 3 −1 X Y x₁ x₂ f(x)
Solve

When the curve crosses the axis, and symmetry

If the curve crosses the $x$-axis inside the interval, the positive and negative parts cancel when you integrate. For the area you have to split into pieces, integrate each one, and add the absolute values.

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Without computing the antiderivative, justify using the properties of the definite integral why the value of the following integral is equal to $0$: $$\int_{-2}^{2} (x^5…

Without computing the antiderivative, justify using the properties of the definite integral why the value of the following integral is equal to $0$:

$$\int_{-2}^{2} (x^5 + \sin(x)) \, dx$$
Solve
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A design studio is designing the logo for a new technology company. The base of the logo is bounded by the function $f(x) = x^3 - 4x$ and the x-axis $OX$. Calculate the…

A design studio is designing the logo for a new technology company. The base of the logo is bounded by the function $f(x) = x^3 - 4x$ and the x-axis $OX$. Calculate the total area of the two enclosed regions formed by the curve and the axis.

x y −2 0 2 A₁ A₂ f(x) = x³ - 4x
Solve

Area between two curves

If the region is bounded by $f$ and $g$ instead of the $x$-axis, the idea is the same but with a subtraction: at each $x$, the height of the region is the distance between the two curves.

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Find the area of the region bounded between the graphs of the functions $f(x) = 6 - x^2$ and $g(x) = 2$. <br>

Find the area of the region bounded between the graphs of the functions $f(x) = 6 - x^2$ and $g(x) = 2$.


x y 0 −2 2 f(x) = 6 - x² g(x) = 2 (−2, 2) (2, 2)
Solve
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Calculate the area of the bounded plane region enclosed by the graphs of the functions $f(x) = -x^2 + 5x$ and $g(x) = x + 2$. Sketch the region before performing the…

Calculate the area of the bounded plane region enclosed by the graphs of the functions $f(x) = -x^2 + 5x$ and $g(x) = x + 2$. Sketch the region before performing the calculation.

0 1 2 3 4 5 2 4 6 f(x) g(x) S x₁ x₂

You can do it on paper and send a photo with your answer.

Solve
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Calculate the area of the region in the first quadrant enclosed by the graphs of $f(x) = \frac{8}{x}$, the line $y = x$ and the line $y = 4$.

Calculate the area of the region in the first quadrant enclosed by the graphs of $f(x) = \frac{8}{x}$, the line $y = x$ and the line $y = 4$.

0 2 √8 4 √8 4 y = 4 y = x f(x) = 8/x
Solve

Regions with tangents, exponentials, and trigonometric functions

Exam regions mix different curves. The method doesn't change: draw it, find the intersections (including axes and any given vertical lines), decide which curve is on top, and on each piece integrate "top minus bottom."

If you are given a tangent line, you get it first: the slope is $f'(x_0)$ and the line is $y-f(x_0)=f'(x_0)(x-x_0)$.

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Find the area of the region bounded by the curve $f(x) = x^2 - 4x + 5$, the tangent line to that curve at the point $(3, f(3))$ and the y-axis $OY$.

Find the area of the region bounded by the curve $f(x) = x^2 - 4x + 5$, the tangent line to that curve at the point $(3, f(3))$ and the y-axis $OY$.

X Y 0 1 2 3 f(x) = x² - 4x + 5 y = 2x - 4 (0, 5) (0, −4) (3, 2) Area
Solve
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Consider the region bounded by the curves $y = e^x$, $y = e^{-x}$ and the vertical line $x = \ln(2)$. a) Draw a sketch of the region. b) Calculate the area of that…

Consider the region bounded by the curves $y = e^x$, $y = e^{-x}$ and the vertical line $x = \ln(2)$.

  1. a) Draw a sketch of the region.
  1. b) Calculate the area of that region.

You can do it on paper and send a photo with your answer.

Solve
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A graphic design company creates a logo shaped like the region bounded by the graphs of the functions $f(x) = \sin(x)$ and $g(x) = \cos(x)$ and by the vertical lines…

A graphic design company creates a logo shaped like the region bounded by the graphs of the functions $f(x) = \sin(x)$ and $g(x) = \cos(x)$ and by the vertical lines $x = 0$ and $x = \pi$ (that is, on the interval $x \in [0, \pi]$). The figure shows both graphs and the shaded logo.

  1. a) Determine the points where the two functions intersect in that interval.
  1. b) The logo is divided into two regions by the x-axis $OX$. Calculate the area of the part of the logo that lies above the $OX$ axis.
  1. c) Justify whether the whole logo (the region between the two graphs on $[0, \pi]$) is symmetric with respect to some vertical line $x = a$ with $a \in [0, \pi]$.
Graphs of sine and cosine on the interval from 0 to pi, with the region between them shaded and divided by the x-axis π/4 π/2 3π/4 π 1 −1 O x y f(x) = sin(x) g(x) = cos(x)
Solve

The integral as accumulated quantity

If a quantity changes at a certain rate, the total accumulated between two instants is the integral of that rate.

Graph of the flow rate of water entering a water tank in liters per minute, with the area shaded between t = 0 and t = 5 minutes, representing the total liters accumulated. Accumulated liters 0 5 min flow rate (L/min)
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The velocity of a radio-controlled car (in $m/s$) is given by the function $v(t) = 2t + 1$. Determine the total distance travelled by the car between the times $t = 1$ s…

The velocity of a radio-controlled car (in $m/s$) is given by the function $v(t) = 2t + 1$. Determine the total distance travelled by the car between the times $t = 1$ s and $t = 4$ s by calculating the definite integral of the velocity.

Solve
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An electronics company has determined that the marginal cost of producing $x$ units of a sensor is $C'(x) = 60x^2 - 240x + 300$ pesos per unit. Calculate the total…

An electronics company has determined that the marginal cost of producing $x$ units of a sensor is $C'(x) = 60x^2 - 240x + 300$ pesos per unit. Calculate the total increase in cost when increasing production from $10$ to $20$ sensors.

Solve

Total distance and displacement

If the velocity $v(t)$ changes sign, the object goes back and forth. Then $\int_a^b v(t)\,dt$ is the displacement (final position minus initial position), and $\int_a^b |v(t)|\,dt$ is the total distance traveled.

For the distance: find the instants where $v=0$, split the interval, integrate each piece, and add the absolute values.

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An autonomous vehicle travels in a straight line and its speed (in m/s) is given by the function $v(t) = t^2 - 6t + 8$, where $t$ is the time in seconds. Calculate the…

An autonomous vehicle travels in a straight line and its speed (in m/s) is given by the function $v(t) = t^2 - 6t + 8$, where $t$ is the time in seconds. Calculate the total distance travelled by the vehicle between time $t = 0$ and $t = 5$. Note: Remember that the distance is the integral of the modulus of the velocity.

Solve
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A surveillance drone moves in a straight line. Its velocity in meters per second is given by the absolute value function: $v(t) = |t^2 - 6t + 5|$ for $t \in [0, 6]$…

A surveillance drone moves in a straight line. Its velocity in meters per second is given by the absolute value function:

$v(t) = |t^2 - 6t + 5|$ for $t \in [0, 6]$ seconds.

  1. a) Determine the total distance traveled by the drone during the first 6 seconds.
  2. b) Calculate the value of the time $T \in (0, 6)$ such that the distance traveled in the interval $[0, T]$ is exactly half the total distance calculated in the previous part.
1 2 3 4 5 6 1 2 3 4 5 0 t (s) v(t) (m/s) v(t) = |t² - 6t + 5| d₁ d₂ d₃
Solve

Functions defined by an integral

If $F(x)=\int_a^x f(t)\,dt$ with $f$ continuous, the Fundamental Theorem says $F'(x)=f(x)$. You get the derivative without computing any antiderivative.

For the tangent line to $F$ at $x_0$ you need the slope $F'(x_0)=f(x_0)$ and the value $F(x_0)$. The latter is usually easy if $x_0$ coincides with the lower limit (then $F=0$) or if, after substituting, the two limits are equal.

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Given the integral function $F(x) = \int_{0}^{x} \sqrt{t^2 + 9} \, dt$, use the Fundamental Theorem of Calculus to find the value of its derivative at the point $x = 4$,…

Given the integral function $F(x) = \int_{0}^{x} \sqrt{t^2 + 9} \, dt$, use the Fundamental Theorem of Calculus to find the value of its derivative at the point $x = 4$, that is, calculate $F'(4)$.

Solve

Variable upper limit: the chain rule

With $F'$ you can now analyze $F$: where $F'>0$ it increases, where $F'<0$ it decreases; the local extrema are where $F'$ changes sign; and the sign of $F''$ gives the concavity (concave up if $F''>0$, concave down if $F''<0$). Inflection points are where $F''$ changes sign.

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Let the function $F(x)$ be defined by the following integral: $F(x) = \int_{2}^{x^2-1} \frac{\ln(t)}{t} dt$ Find the equation of the tangent line to the graph of $F(x)$…

Let the function $F(x)$ be defined by the following integral:

$F(x) = \int_{2}^{x^2-1} \frac{\ln(t)}{t} dt$

Find the equation of the tangent line to the graph of $F(x)$ at the point with x-coordinate $x = \sqrt{3}$.

Solve
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Let the function $f(x)$ be defined by the following expression involving an integral: $F(x) = \int_{0}^{x^2} \frac{t-1}{2+e^t} dt$ Determine, giving reasons: a) The…

Let the function $f(x)$ be defined by the following expression involving an integral:

$F(x) = \int_{0}^{x^2} \frac{t-1}{2+e^t} dt$

Determine, giving reasons:

  1. a) The intervals of increase and decrease of $F(x)$ across its entire domain.
  2. b) The x-coordinates of the points where $F(x)$ reaches its local extrema, classifying them as maximums or minimums.
  3. c) Does the function $F(x)$ have any point of inflection in the interval $(0, +\infty)$? Justify your answer using the second derivative.
t y 0 1 x² g(t) = (t-1)/(2+eᵗ) F(x) = ∫₀ˣ² g(t) dt - +
Solve

Finding a parameter from an area

When you are given the value of the area and asked for a constant, the method is: set up the integral with the unknown inside, apply Barrow's rule, and set it equal to the given value. The limits often depend on the parameter, because they are the intersection points.

Parabola y = x squared and horizontal line y = c, with the region between them shaded symmetrically between the intersections minus square root of c and square root of c. −√c √c y = c Given area
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Determine the value of the positive parameter $k > 0$ such that the area of the region bounded by the parabola $y = kx^2$ and the line $y = 4$ is exactly $16$ square…

Determine the value of the positive parameter $k > 0$ such that the area of the region bounded by the parabola $y = kx^2$ and the line $y = 4$ is exactly $16$ square units.

Solve
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A team of engineers is designing the entrance archway to a technology tunnel. The shape of the upper arch is given by the function $f(x) = 4 - x^2$ for $x \in [-2, 2]$.…

A team of engineers is designing the entrance archway to a technology tunnel. The shape of the upper arch is given by the function $f(x) = 4 - x^2$ for $x \in [-2, 2]$. To reinforce the structure, a straight line $y = k$ (where $0 < k < 4$) must be drawn such that the area of the upper section (above the reinforcing line) is exactly twice the area of the lower section (between the reinforcing line and the ground, located on the $OX$ axis).

Calculate the exact value of the constant $k$ that satisfies this condition.

X Y −2 2 4 0 f(x) = 4 - x² y = k A₁ A₂ A₁ = 2A₂
Solve

One more step: volume of revolution

If you rotate a region around an axis, you get a solid. Imagine slicing it into thin disks: each one has volume $\pi\,(\text{radius})^2\,dx$.

Horizontal asymptote

Sometimes the line of rotation is the horizontal asymptote of the function (this is the case for $\dfrac{x^2}{x^2+1}$ in one of the exercises), so it is useful to know how to find it.

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Consider the region of the plane $R$ bounded by the curve $y = \ln(x)$, the tangent line to this curve at the point with x-coordinate $x = e$, and the x-axis $OX$. a)…

Consider the region of the plane $R$ bounded by the curve $y = \ln(x)$, the tangent line to this curve at the point with x-coordinate $x = e$, and the x-axis $OX$.

  1. a) Find the equation of the aforementioned tangent line.
  2. b) Calculate the area of the region $R$.
  3. c) Calculate the volume of the solid of revolution generated by rotating the region $R$ about the $OX$ axis.
X Y (e, 1) e 1 O y = ln(x) Tangent: y = ? R (Area = ?) Volume = ? Rotation about OX
Solve
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A metal piece is to be manufactured whose cross section is bounded by the curves $y = \sqrt{x}$, $y = x - 2$ and the $y$-axis $OY$. a) Sketch the region and determine…

A metal piece is to be manufactured whose cross section is bounded by the curves $y = \sqrt{x}$, $y = x - 2$ and the $y$-axis $OY$.

  1. a) Sketch the region and determine its points of intersection.
  1. b) Calculate the area of that cross section.
  1. c) If the piece is obtained by rotating this region about the $y$-axis $OY$, set up (without needing to calculate) the integral or sum of integrals that would give the volume of the piece.

You can do it on paper and send a photo with your answer.

Solve
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Given the function $f(x) = \frac{x^2}{x^2 + 1}$, consider the region bounded by the graph of $f(x)$, its horizontal asymptote and the vertical lines $x = -1$ and…

Given the function $f(x) = \frac{x^2}{x^2 + 1}$, consider the region bounded by the graph of $f(x)$, its horizontal asymptote and the vertical lines $x = -1$ and $x = 1$.

  1. a) Identify the equation of the horizontal asymptote of the function.
  2. b) Calculate the area of this region.
  3. c) Determine the volume of the solid generated by rotating this region around its horizontal asymptote.

(Hint: Perform a change of variable or translation to facilitate the calculation of the volume).

x y 0 x = −1 x = 1 y = ? f(x) Area = ? Volume = ?
Solve

One more step: limits with integrals and L'Hôpital

If a limit gives $\frac00$ and the numerator or the denominator contains an integral, you can differentiate both. L'Hôpital: if $\lim \frac{f}{g}$ gives $\frac00$ (with $f,g$ differentiable and $g'\ne0$ near the point), then $\lim\frac{f}{g}=\lim\frac{f'}{g'}$ when the latter exists. The derivative of the integral comes from the Fundamental Theorem.

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Calculate the value of the following limit applying the Fundamental Theorem of Calculus and L'Hôpital's Rule:…

Calculate the value of the following limit applying the Fundamental Theorem of Calculus and L'Hôpital's Rule:

$\lim_{x \to 0} \frac{x \times \int_{0}^{x} e^{-t^2} dt}{1 - \cos(x^2)}$

State in detail the steps followed and the necessary hypotheses for applying each theorem.

Solve

Summary

Go back to the beginning: the liters that enter the tank are the integral of the flow rate between the two times, just as distance is the integral of velocity.

See all the exercises in this section