AP Calculus AB : Integrals

Study concepts, example questions & explanations for AP Calculus AB

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Example Questions

Example Question #41 : Integrals

Solve the separable differential equation

\(\displaystyle \frac{\mathrm{d} y}{\mathrm{d} x} = y(2x+x^2)\)

given the initial condition

\(\displaystyle f(0)=14\)

 

Possible Answers:

\(\displaystyle y=14e^{2x+\frac{x^3}{3}}\)

\(\displaystyle y=Ce^{x^2+\frac{x^3}{3}}\)

\(\displaystyle y=14e^{x^2+\frac{x^3}{3}}\)

\(\displaystyle y=14e^{\frac{x^2}{2}+\frac{x^3}{3}}\)

\(\displaystyle y=14e^{x^2+\frac{x^2}{3}}\)

Correct answer:

\(\displaystyle y=14e^{x^2+\frac{x^3}{3}}\)

Explanation:

To solve the separable differential equation, we must separate x and y to the same sides as their respective derivatives:

\(\displaystyle \frac{dy}{y} = (2x+x^2)dx\)

Next, we integrate both sides:

\(\displaystyle \ln \left | y\right |= x^2+\frac{x^3}{3}+C\)

The integrals were solved using the following rules:

\(\displaystyle \int \frac{dx}{x}=\ln \left | x\right |+C\)\(\displaystyle \int x^n dx = \frac{x^{n+1}}{n+1}+C\)

The two constants of integration were combined to make a single constant.

Now, exponentiate both sides to isolate y, and use the properties of exponents to rearrange the integration constant:

\(\displaystyle e^{\ln\left | y\right |}= e^{x^2+\frac{x^3}{3}+C}\)

\(\displaystyle y=e^{x^2+\frac{x^3}{3}}\cdot e^C=Ce^{x^2+\frac{x^3}{3}}\)

(The exponential of the constant is another constant.)

Finally, we solve for the integration constant using the initial condition:

\(\displaystyle 14=Ce^0=C\)

Our final answer is

\(\displaystyle y=14e^{x^2+\frac{x^3}{3}}\)

Example Question #42 : Integrals

Solve the separable differential equation:

\(\displaystyle \frac{\mathrm{d} y}{\mathrm{d} x} = \frac{x^4+e^x}{2y}, y< 0\)

Possible Answers:

\(\displaystyle y=\sqrt{\frac{x^5}{5}+e^x+C}\)

\(\displaystyle y=\sqrt{\frac{x^5}{5}+e^x}+C\)

\(\displaystyle y=-\sqrt{\frac{x^5}{5}+e^x+C}\)

\(\displaystyle y=-\sqrt{{x^5}+e^x+C}\)

Correct answer:

\(\displaystyle y=-\sqrt{\frac{x^5}{5}+e^x+C}\)

Explanation:

To solve the separable differential equation, we must separate x and y to the same sides as their respective derivatives:

\(\displaystyle 2y dy=(x^4+x)dx\)

Next, we integrate both sides:

\(\displaystyle y^2=\frac{x^5}{5}+e^x+C\)

The integrals were solved using the following rules:

\(\displaystyle \int x^ndx=\frac{x^{n+1}}{n+1}+C\)\(\displaystyle \int e^x dx=e^x+C\)

The two constants of integration were combined to make a single one.

Now, we solve for y:

\(\displaystyle y=\pm \sqrt{\frac{x^5}{5}+e^x+C}\)

Because the problem statement said that y is negative - and y cannot be zero - our final answer is

\(\displaystyle y=-\sqrt{\frac{x^5}{5}+e^x+C}\)

Example Question #43 : Integrals

Solve the separable differential equation:

\(\displaystyle \frac{\mathrm{d} y}{\mathrm{d} x} = \frac{4y}{x^2}, x>0\)

and at \(\displaystyle x=1, y=2\)

Possible Answers:

\(\displaystyle y=Ce^{-\frac{4}{x}}\)

\(\displaystyle y=2e^{-\frac{4}{x}}\)

\(\displaystyle y=2e^{-\frac{4}{x}}+C\)

\(\displaystyle y=2e^{4-\frac{4}{x}}\)

\(\displaystyle y=2\)

Correct answer:

\(\displaystyle y=2e^{4-\frac{4}{x}}\)

Explanation:

To solve the separable differential equation, we must separate x and y to the same sides as their respective derivatives:

\(\displaystyle \frac{dy}{y}=\frac{dx}{x^2}\)

Next, we integrate both sides:

\(\displaystyle \ln \left | y\right | = -\frac{1}{x}+C\)

The integrals were solved using the following rules:

\(\displaystyle \int \frac{dx}{x}= \ln \left | x\right |+C\)\(\displaystyle \int x^n dx = \frac{x^{n+1}}{n+1}+C\)

The two constants of integration were combined to make a single one.

Now, we exponentiate both sides to solve for y:

\(\displaystyle y=e^{-\frac{1}{x}+C}\)

Using the properties of exponents, we can rearrange the integration constant:

\(\displaystyle y=e^{-\frac{1}{x}+C} = e^{-\frac{1}{x}}\cdot e^C = Ce^{-\frac{1}{x}}\)

(The exponential of the constant is itself a constant.)

Using the given condition, we can solve for C:

\(\displaystyle 2=Ce^{-4}\)

\(\displaystyle C=2e^4\)

Our final answer is

\(\displaystyle y=2e^{4-\frac{4}{x}}\)

 

 

Example Question #44 : Integrals

The rate of a chemical reaction is given by the following differential equation:

\(\displaystyle \frac{-d[A]}{dt} = k[A]^{2}\),

where \(\displaystyle \left [ A \right ]\) is the concentration of compound \(\displaystyle A\) at a given time, \(\displaystyle t\).  Which one of the following equations describes \(\displaystyle \frac{1}{\left [ A \right ]}\) as a function of time?  Let \(\displaystyle \left [ A \right ]_{0}\) be the concentration of compound \(\displaystyle A\) when \(\displaystyle t=0\).

Possible Answers:

\(\displaystyle \frac{1}{[A_{0}]} + kt = \frac{1}{[A]}\)

\(\displaystyle [A_{0}]kt= \frac{1}{[A]}\)

\(\displaystyle \frac{1}{[A_{0}]} - kt = \frac{1}{[A]}\)

\(\displaystyle [A_{0}]e^{-kt}= \frac{1}{[A]}\)

\(\displaystyle [A_{0}]e^{kt}= \frac{1}{[A]}\)

Correct answer:

\(\displaystyle \frac{1}{[A_{0}]} + kt = \frac{1}{[A]}\)

Explanation:

To begin with, the differential equation needs to be rearranged so that each variable is one side of the equation:

\(\displaystyle \frac{-d[A]}{dt} = k[A]^{2} \Rightarrow \frac{d[A]}{[A]^{2}} = -kdt\).  

Then, integrate each side of the rate law, bearing in mind that \(\displaystyle \left [ A \right ]\) will range from \(\displaystyle \left [ A \right ]_{0}\) to \(\displaystyle \left [ A \right ]\), and time will range from \(\displaystyle 0\) to \(\displaystyle t\):

\(\displaystyle \int_{[A]_{0}}^{[A]}\frac{d[A]}{[A]^{2}} = \int_{0}^{t} -kdt\)

After integrating each side, the equation becomes:

\(\displaystyle \frac{-1}{[A]} = -kt\).  

The left side has to be evaluated from \(\displaystyle \left [ A \right ]_{0}\) to \(\displaystyle \left [ A \right ]\), and the right side is evaluated from \(\displaystyle 0\) to \(\displaystyle t\):

\(\displaystyle \frac{-1}{[A]} - ( \frac{-1}{[A_{0}]}) = -kt - (-k(0))\).  This becomes:

\(\displaystyle \frac{1}{[A_{0}]} - \frac{1}{[A]} = -kt\).  

Finally, rearranging gives:

\(\displaystyle \frac{1}{[A_{0}]} + kt = \frac{1}{[A]}\)

Example Question #45 : Integrals

Given that \(\displaystyle y' = (2y + 2)(x -2)\) and \(\displaystyle y(0) = 1\), solve for \(\displaystyle y(t)\). What is the value of \(\displaystyle y(4)\)?

Possible Answers:

1

\(\displaystyle 2e - 1\)

\(\displaystyle -1\)

\(\displaystyle 64\)

\(\displaystyle 2e^2 - 1\)

Correct answer:

1

Explanation:

This is a separable differential equation. The simplest way to approach this is to turn \(\displaystyle y'\) into \(\displaystyle \frac{\mathrm{d} y}{\mathrm{d} x}\), and then by abusing the notation, "multiplying by dx" on both sides.

\(\displaystyle dy = (2y+2)(x - 2)dx\)

We then group all the y terms with dy, and all the x terms with dx.

\(\displaystyle \frac{1}{2y+2}dy = (x - 2)dx\)

Integrating both sides, we find 

\(\displaystyle \int \frac{1}{2y+2}dy = \frac{\ln|2y+2|}{2} =\int (x - 2)dx = \frac{x^2}{2} - 2x + C\)

Here, the first integral is found by using substitution of variables, setting \(\displaystyle u = 2y +2\). In addition, we have chosen to only put a +C on the second integral, as if we put it on both, we would just combine them in any case.

To solve for y, we multiply both sides by two and raise e to both sides to get rid of the natural logarithm.

\(\displaystyle |2y+2| = e^{x^2 - 4x + C}\)

(Note, C was multiplied by two, but it's still just an arbitrary constant. If you prefer, you may call the new C value \(\displaystyle C_1\).)

Now we drop our absolute value signs, and note that we can take out a factor of \(\displaystyle e^C\) and stick in front of the right hand side.

\(\displaystyle 2y+2 =\pm e^Ce^{x^2 - 4x}\)

As \(\displaystyle \pm e^C\) is just another arbitrary constant, we can relabel this as C, or \(\displaystyle C_2\) if you prefer. Solving for y gets us

\(\displaystyle y = \frac{Ce^{x^2 - 4x} - 2}{2}\)

Next, we plug in our initial condition to solve for C.

\(\displaystyle 1 =\frac{C - 2}{2}\)\(\displaystyle C = 4\)

Leaving us with a final equation of

\(\displaystyle y = \frac{4e^{x^2 - 4x} - 2}{2}\)

Plugging in x = 4, we have a final answer,

\(\displaystyle y = \frac{4e^{16 - 16} - 2}{2} = \frac{4 - 2}{2} = 1\)

Example Question #46 : Integrals

Solve the separable differential equation:

\(\displaystyle \frac{\mathrm{d} y}{\mathrm{d} x}=y\cos(x)\)

given the condition at

\(\displaystyle x=\pi, y=2\)

Possible Answers:

\(\displaystyle y=x\)

\(\displaystyle y=Ce^{\sin(x)}\)

\(\displaystyle y=e^{\sin(x)}\)

\(\displaystyle y=2e^{\sin(x)}\)

Correct answer:

\(\displaystyle y=2e^{\sin(x)}\)

Explanation:

To solve the separable differential equation, we must separate x and y to the same sides as their respective derivatives:

\(\displaystyle \frac{dy}{y}=\cos(x)dx\)

Next, we integrate both sides:

\(\displaystyle \int \frac{dy}{y}=\ln\left | y\right |+C\)\(\displaystyle \int \cos(x)dx=\sin(x)+C\)

The integrals were solved using the identical rules.

The two constants of integration are now combined to make a single one:

\(\displaystyle \ln \left | y\right |= \sin(x)+C\)

Now, exponentiate both sides of the equation to solve for y, and use the properties of exponents to rearrange C:

\(\displaystyle y=e^{\sin(x)+C}=e^{\sin(x)}\cdot e^C=Ce^{\sin(x)}\)

Finally, we solve for the integration constant using the given condition:

\(\displaystyle 2=Ce^{\sin(\pi)}=Ce^0=C\)

Our final answer is

\(\displaystyle y=2e^{\sin(x)}\)

Example Question #47 : Integrals

If f(1) = 12, f' is continuous, and the integral from 1 to 4 of f'(x)dx = 16, what is the value of f(4)?

Possible Answers:

4

27

16

28

12

Correct answer:

28

Explanation:

You are provided f(1) and are told to find the value of f(4). By the FTC, the following follows:

(integral from 1 to 4 of f'(x)dx) + f(1) = f(4)

16 + 12 = 28

Example Question #1 : Interpretations And Properties Of Definite Integrals

Find the limit. 

lim as n approaches infiniti of ((4n3) – 6n)/((n3) – 2n+ 6)

Possible Answers:

nonexistent

–6

0

4

1

Correct answer:

4

Explanation:

lim as n approaches infiniti of ((4n3) – 6n)/((n3) – 2n+ 6)

Use L'Hopitals rule to find the limit. 

lim as n approaches infiniti of ((4n3) – 6n)/((n3) – 2n+ 6)

lim as n approaches infiniti of ((12n2) – 6)/((3n2) – 4n + 6)

lim as n approaches infiniti of 24n/(6n – 4)

lim as n approaches infiniti of 24/6

The limit approaches 4. 

Example Question #2 : Definite Integral Of The Rate Of Change Of A Quantity Over An Interval Interpreted As The Change Of The Quantity Over The Interval

If a particle's movement is represented by p=3t^{2}-t+16\(\displaystyle p=3t^{2}-t+16\), then when is the velocity equal to zero?

Possible Answers:

\(\displaystyle 1\)

\(\displaystyle \frac{1}{6}\)

\(\displaystyle 16\)

\(\displaystyle 0\)

\(\displaystyle 6\)

Correct answer:

\(\displaystyle \frac{1}{6}\)

Explanation:

The answer is \(\displaystyle \frac{1}{6}\) seconds.

 

p=3t^{2}-t+16\(\displaystyle p=3t^{2}-t+16\)

v=p'=6t-1\(\displaystyle v=p'=6t-1\)

now set \(\displaystyle v=0\) because that is what the question is asking for. 

v=0=6t-1\(\displaystyle v=0=6t-1\)

t=\frac{1}{6}\(\displaystyle t=\frac{1}{6}\) seconds

Example Question #3 : Definite Integral Of The Rate Of Change Of A Quantity Over An Interval Interpreted As The Change Of The Quantity Over The Interval

A particle's movement is represented by p=-t^{2}+12t+2\(\displaystyle p=-t^{2}+12t+2\)

 At what time is the velocity at it's greatest?

Possible Answers:

\(\displaystyle 12\)

\(\displaystyle -2\)

\(\displaystyle 6\)

\(\displaystyle 0\)

\(\displaystyle 2\)

Correct answer:

\(\displaystyle 6\)

Explanation:

The answer is at 6 seconds. 

 

p=-t^{2}+12t+2\(\displaystyle p=-t^{2}+12t+2\) 

We can see that this equation will look like a upside down parabola so we know there will be only one maximum.

v=p'=-2t+12\(\displaystyle v=p'=-2t+12\)

Now we set \(\displaystyle v=0\) to find the local maximum. 

v=0=-2t+12\(\displaystyle v=0=-2t+12\)

t=6\(\displaystyle t=6\) seconds

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