Calculus 1 : How to find integral expressions

Study concepts, example questions & explanations for Calculus 1

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Example Questions

Example Question #181 : Writing Equations

Solve the following integral, where a and b are constants:

\(\displaystyle \int abe^{b^2x}dx\)

Possible Answers:

\(\displaystyle \frac{ae^{b^2x}}{b}+C\)

\(\displaystyle \frac{1}{2}ae^{b^2x}+C\)

\(\displaystyle ae^{b^2x}+C\)

\(\displaystyle \frac{2ae^{b^2x}}{b}+C\)

Correct answer:

\(\displaystyle \frac{ae^{b^2x}}{b}+C\)

Explanation:

Keeping in mind that a and b are only constants, the integral is equal to

\(\displaystyle \int abe^{b^2x}dx=\frac{abe^{b^2x}}{b^2}+C=\frac{ae^{b^2x}}{b}+C\)

and was found using the following rule:

\(\displaystyle \int e^{ax}dx=\frac{e^{ax}}{a}+C\)

Example Question #182 : How To Find Integral Expressions

Evaluate the following integral:

\(\displaystyle \int xy\cos(x^2y)dx\)

Possible Answers:

\(\displaystyle \frac{1}{2}\sin(x^2y)+\frac{x^2y}{2}+C\)

\(\displaystyle \sin(x^2y)+C\)

\(\displaystyle -\frac{1}{2}\sin(x^2y)+C\)

\(\displaystyle \frac{1}{2}\sin(x^2y)+C\)

Correct answer:

\(\displaystyle \frac{1}{2}\sin(x^2y)+C\)

Explanation:

To evaluate the integral, we must make the following substitution:

\(\displaystyle u=x^2y\)\(\displaystyle du=2xydx\)

The derivative was found using the following rule:

\(\displaystyle \frac{\mathrm{d} }{\mathrm{d} x} x^n=nx^{n-1}\)

Rewrite the integral in terms of u and integrate:

\(\displaystyle \frac{1}{2}\int \cos(u)du=\frac{1}{2}\sin(u)+C\)

The integral was found using the following rule:

\(\displaystyle \int \cos(x)dx=\sin(x)+C\)

Finally, replace u with our original term:

\(\displaystyle \frac{1}{2}\sin(x^2y)+C\).

Example Question #2261 : Calculus

In circuits with a resistor, the equation for voltage drop is given by:

\(\displaystyle \frac{d}{dx}V =\frac{d}{dt}Q*R\), where \(\displaystyle V\) is voltage, \(\displaystyle Q\) is charge, and \(\displaystyle R\) is resistance. 

Write the equation as an integral expression for \(\displaystyle Q\)

Possible Answers:

\(\displaystyle Q=\int VR dt\)

\(\displaystyle Q=\int \frac{V}{R} dt\)

\(\displaystyle Q=\int {\frac{d}{dx}V}R dt\)

\(\displaystyle Q=\int \frac{\frac{d}{dx}V}{R} dt\)

Correct answer:

\(\displaystyle Q=\int \frac{\frac{d}{dx}V}{R} dt\)

Explanation:

Although this may seem really difficult, we only need to solve for \(\displaystyle Q\)

\(\displaystyle \frac{d}{dx}V =\frac{d}{dt}Q*R\)

\(\displaystyle \frac{d}{dt}Q=\frac{\frac{\mathrm{d} }{\mathrm{d} x}V}{R}\)

To solve for \(\displaystyle Q\), integrate both sides:

\(\displaystyle Q=\int \frac{\frac{d}{dx}V}{R} dt\)

Example Question #182 : Writing Equations

\(\displaystyle \int_{0}^{2}2x^2-5x+1 dx\)

Possible Answers:

\(\displaystyle \frac{8}{3}\)

\(\displaystyle -\frac{5}{3}\)

\(\displaystyle -\frac{11}{3}\)

\(\displaystyle -\frac{8}{3}\)

Correct answer:

\(\displaystyle -\frac{8}{3}\)

Explanation:

When integrating, remember to add one to the exponent and then put that result on the denominator: \(\displaystyle \frac{2x^3}{3}-\frac{5x^2}{2}+x\). Now evaluate at 2, and then 0. Then subtract the two results. \(\displaystyle (\frac{16}{3}-10+2)-0=-\frac{8}{3}\).

Example Question #2265 : Calculus

\(\displaystyle \int \frac{24x^3-2x^2+4x}{4x}dx\)

Possible Answers:

\(\displaystyle 2x^3-\frac{x^2}{4}+x\)

\(\displaystyle 2x^3+\frac{x^2}{4}+x+C\)

\(\displaystyle 2x^3-\frac{x^2}{4}+4x+C\)

\(\displaystyle -2x^3-\frac{x^2}{4}+x+C\)

\(\displaystyle 2x^3-\frac{x^2}{4}+x+C\)

Correct answer:

\(\displaystyle 2x^3-\frac{x^2}{4}+x+C\)

Explanation:

The first step here is to chop this up into three separate terms and then simplify since we have only one denominator: \(\displaystyle \int 6x^2-\frac{1}{2}x+1dx\). Then, integrate each term, remembering to add one to the exponent and then put that result on the denominator: \(\displaystyle 6(\frac{x^3}{3})-\frac{1}{2}(\frac{x^2}{2})+x\). Simplify to get your answer: \(\displaystyle 2x^3-\frac{x^2}{4}+x+C\). Remember to add C because it is an indefinite integral.

Example Question #183 : Integral Expressions

\(\displaystyle \int \frac{9+x}{x}dx\)

Possible Answers:

\(\displaystyle 9ln\left | x \right |+x+C\)

\(\displaystyle 9ln\left | x \right |-x+C\)

\(\displaystyle \frac{1}{9}ln\left | x \right |+x+C\)

\(\displaystyle ln\left | x \right |+x+C\)

Correct answer:

\(\displaystyle 9ln\left | x \right |+x+C\)

Explanation:

First, chop this expression up into two terms: \(\displaystyle \int \frac{9}{x}+1dx\). Then, integrate each term, remembering that when there is a single x on a denominator, the integral is \(\displaystyle ln\left | x \right |\). Therefore, the integration is: \(\displaystyle 9ln\left | x \right |+x+C\). Remember to add C because it is an indefinite integral.

Example Question #183 : Writing Equations

\(\displaystyle \int_{0}^{3}2x^3-3x^2+xdx\)

Possible Answers:

\(\displaystyle -81\)

\(\displaystyle 27\)

\(\displaystyle 18\)

\(\displaystyle -18\)

\(\displaystyle 108\)

Correct answer:

\(\displaystyle 18\)

Explanation:

First, integrate each term separately. Remember, when integrating, raise the exponent by one and then also put that result on the denominator: \(\displaystyle 2(\frac{x^4}{4})-3(\frac{x^3}{3})+\frac{x^2}{2}\). Then evaluate at 3 and then 0. Subtract two results to get: \(\displaystyle (\frac{81}{2}-27+\frac{9}{2})-0=18\).

Example Question #185 : How To Find Integral Expressions

\(\displaystyle \int \sqrt{x}dx\)

Possible Answers:

\(\displaystyle \frac{2}{3}x^{\frac{3}{2}}+C\)

\(\displaystyle \frac{2}{3}\)

\(\displaystyle -\frac{2}{3}x^{\frac{3}{2}}+C\)

\(\displaystyle \frac{2}{3}x^2+C\)

Correct answer:

\(\displaystyle \frac{2}{3}x^{\frac{3}{2}}+C\)

Explanation:

To integrate this expression, I would first rewrite it to be \(\displaystyle \int x^{\frac{1}{2}}dx\). Then, add one to the exponent and then put that result on the denominator: \(\displaystyle \frac{x^{\frac{3}{2}}}{\frac{3}{2}}\). Then, simplify to get your answer of \(\displaystyle \frac{2}{3}x^{\frac{3}{2}}+C\). Remember to add C because it is an indefinite integral.

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