All aspects of integration, including the definition of the integral and computing different types of integrals. For questions solely about the properties of integrals, don't use this tag alone! Use in conjunction with (indefinite-integral), (definite-integral), (improper-integrals) or another ...

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1answer
15 views

Is there a clever way to determine negative area of an integral?

Given some continuous, integratable function f(x) that has only positive area over a range from x1 to x2...is there a way to determine the negative area of the integral of f(x) - c (from x1 to x2), ...
6
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0answers
30 views

Closed form of $\int_0^1\left(\frac{\arctan x}{x}\right)^n\,dx$

Inspired by this question, is there a closed-form of $$\int_0^1\left(\frac{\arctan x}{x}\right)^n\,dx\,?$$ Here $n \in \mathbb{N_+}$. In the answers to the question above we could find proofs of ...
0
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1answer
22 views

Find the Antiderivative f''(x) = 2+ cosx, f(0) = -1, f(pi/2) = 0

I integrated it once to get, 2x + sinx + C, C being a constant. Then I integrated it a second time to get x^2 - cosx + Cx + D, D being another constant. I then have to plug in the values to determine ...
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0answers
10 views

ODE - Laplace transform

I have an ODE $\psi^{'}(s)_{3 \times 3}=(A+Bs)_{3 \times 3}\psi(s)_{3 \times 3} \tag1$ where A,B are constant skew symmetric matrices with zero determinant. $\psi(s)$ is a rotation matrix. It implies ...
2
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1answer
34 views

Compute a multiple integral$\iint_{[0,1]^2} (xy)^{xy} dxdy$

$$\text{Compute} :\iint_{[0,1]^2} (xy)^{xy} dxdy$$ I am thinking about changing the variable, $x=u,y={v \over u}$.But it doesn't work. I just found that the answer is$\int_0^1 t^t dt$.Maybe my idea ...
2
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0answers
20 views

If $f$ is increasing, then for all $n\in\mathbb{N}$ there exists $P_n$ : $U(f,P)-L(f,P) \leq (b-a)/n$

I've already proven that, if $f:[a,b] \to \mathbb{R}$ is continuous and increasing, with $a,b\in \mathbb{R}$, then $$U(f,P) - L(f,P) = \sum_{i=1}^{n}\left[ f(x_i) - f(x_{i-1})\right](x_i - x_{i-1})$$ ...
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0answers
12 views

I want to compute the Fourier Transform of $g(x) = (1+e^{a x})^{1/a} \mathbf{1}_{x<0} e^{-x}$

I would like to compute the fourier transform of $g(x) = (1+e^{a x})^{1/a} \mathbf{1}_{x<0}\ e^{-x}$ Question 1: Does the transform exist in any sense?. The sufficient condition ...
0
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1answer
6 views

Joint CDF from conditional cdf

I would like to derive an expression of the following joint CDF $P[X \leq x,Y \leq y]$ based on the conditional CDF $P[X \leq x | Y=y]$ and the pdf $P[Y=y]$ that are considered to be known. I get a ...
3
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3answers
51 views

How to compute $ \int e^{-st} \sin(2t) dt $

Wolfram Alpha shows me the result of $ \int e^{-st} \sin(2t) dt $ . However it doesn't let me see the step to step solution. Then I tried to do this by hand as the solution didn't look "too ...
1
vote
1answer
19 views

Application of the mean value theorem for Integrals

Suppose that $f(x)$ is a differentiable function in $[a,b]$, $f^{'}(x)$ is a monotone decreasing function in $(a,b)$, and $f^{'}(b)>0$. So how to prove that $$ \big \vert \int_a^b \cos ...
1
vote
1answer
44 views

How to integrate $e^{\sqrt{2x}}$?

I think this problem requires integration by substitution and integration by parts, but I seem to get stuck each time I try to solve it. And I'm not sure whether '$u$' should be equal to $\sqrt{2}$ or ...
1
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2answers
105 views

Differentiating with respect to the limit of integration

I'm confused about problems involving differentiation with respect to the limit of an integral, I just want to check that my understanding is correct. For example, are the following statements ...
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0answers
51 views

What is the recurrence relation between $a_n, a_{n-1}$ , $a_n = \int_0^1 {x^n}\tan\left( \frac{\pi}{4}x\right) dx$

I would appreciate if somebody could help me with the following problem Q: What is the recurrence relation between $a_n, a_{n-1}$ ? $$ a_n = \int_0^1 {x^n}\tan\left( \frac{\pi}{4}x\right) dx,\ \ ...
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0answers
36 views

Can this be expressed by a contour integral?

Let $f(z)$ be a real entire function of the form $f(z) = a_1 z + a_2 z^2 + ...$ such that $0 < a_{n+1} < a_n$. Consider $g(x) = f^{-1}(f(x)-f(x-1))$ where $x$ is a positive real and $f^{-1}$ ...
7
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3answers
68 views

Improper integral : $\int_0^{+\infty}\frac{x\sin x}{x^2+1}$

How to evaluate the following improper integral : $$\int_0^{+\infty}\frac{x\sin x}{x^2+1}\,dx$$ I have tried integration by parts and variable substitution, but it didn't work.
2
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4answers
100 views

Inverse Trigonometric Integrals

How to calculate the value of the integrals $$\int_0^1\left(\frac{\arctan x}{x}\right)^2\,dx,$$ $$\int_0^1\left(\frac{\arctan x}{x}\right)^3\,dx $$ and $$\int_0^1\frac{\arctan^2 x\ln x}{x}\,dx?$$
3
votes
3answers
72 views

Evaluate integral: $\int_0^{+\infty}\frac{\cos{bx}-\cos{ax}}{x}dx$

It seems that $\displaystyle\int_0^{+\infty}\frac{\cos x}{x}$ is divergent, so how to solve this problem? $$\int_0^\infty \frac{\cos bx -\cos ax}{x}\, dx\quad,\quad\mbox{where}\,a,b>0$$ It's ...
0
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1answer
35 views

Calculate integral of $\ln(z)$ using the residue theorem

Please is it possible to calculate $\int_{C(0,1)}\ln(z)\,dz$ using the residue theorem? Thank you for your help.
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0answers
14 views

Calculating volumes using integral.

Given $y=x,y=0,x=2$ and $x=7$. Calculate the volume6 about $x=1$. I just need to get the concept right. Please tell me what mistake I did here. The region looks like a trapezium right? From $y=0$ ...
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1answer
12 views

Integral of a normal function multiplied by heaviside and delta functions

$\int_{-\infty}^{\infty} e^{2t}u(\tau - t)t^{2}\delta(t)dt$ Hi! How would I go about computing this integral? I understand I can change one of the integration limits and eliminate the heaviside ...
6
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0answers
45 views

Closed form of a difficult definite integral

I'm looking for a closed-form expression for the value of this integral: $$I=\int_0^\pi \frac{\sin(x)}{\sqrt{x^3+x+1}} dx$$ The graph of the integrand looks like this: $\hskip 2.4 in$ Numerically, ...
1
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5answers
616 views

How can I show that these integrals are zero

How can I show that these integrals equal $0$ when $n$ and $m$ are both integers and $n \neq m$? $$\int_{-\pi}^{\pi}\sin(mx)\sin(nx)dx = \int_{-\pi}^{\pi}\cos(mx)\cos(nx)dx = 0$$ I'm able to show that ...
2
votes
3answers
69 views

If $f'(x)=f(x)+\int_{0}^{1}f(x)\,dx$ and $f(0) = 1,\,$ then what is the value of $\, \int_0^1 f(x)\,dx=$?

If $\displaystyle f'(x)=f(x)+\int_{0}^{1}f(x)\,dx\,$ and $\,f(0) = 1.$ Then what is value of $\displaystyle \int f(x)\,dx\,?$ $\bf{My\; Try.}$ Let $\displaystyle \int_{0}^{1}f(x)\,dx = A\;,$ Then ...
1
vote
1answer
32 views

Any Easier way to integrate:$\iint\limits_D{e^{x+y}}d\sigma,D=\{\left . (x,y) \right ||x|+|y|\leqslant1\}$

This is my way: \begin{align} \iint\limits_D{e^{x+y}}d\sigma & = \int_{-1}^0e^xdx\int_{-x-1}^{x+1}e^ydy + \int_0^1e^xdx\int_{x-1}^{-x+1}e^ydy \\ & = \cdots \\ & = e-e^{-1} ...
1
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0answers
30 views

How do I do this double integral (change of variable)

$B$ is the region bounded by $xy = 1$, $xy = 3$, $x^2 - y^2 = 1$, $x^2 - y^2 = 4$ Find $$\iint\limits_{B}x^2 + y^2 \,dx\,dy$$ using the change of variables: $$u = x^2 - y^2$$ $$v = xy$$ So I think ...
4
votes
1answer
49 views

Is there an alternative way to solve this integral?

I was given the integral $$\int \frac{2}{e^{-x}+1}dx$$ Here is my method to get the (correct) solution: $$\int \frac{2}{e^{-x}+1}dx$$ $$=2\int \frac{1}{e^{-x}+1}dx$$ $$=2\int ...
0
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0answers
19 views

Complex Fourier coefficients and series

I need help trying to find the complex Fourier coefficients for the functions $\cos(3x)$ $\sin(2x)$ I know the equation for finding the coefficients and how to plug it in but I'm confused in how ...
1
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3answers
65 views

Fallacy - where is the mistake?

Could anyone help me to find the mistake in this fallacy? Because the actual result for $I$ is $\pi/2$ \begin{equation} I = \int_{0}^{\pi} \cos^{2} x \; \textrm{d}x \end{equation} \begin{equation} I ...
1
vote
1answer
20 views

Solution to Differential Equation $\left( 1-2\lambda\frac{\partial}{\partial z}\right)w(x,y,z)-g(x,y,z+h)+2 \lambda h(x,y,z)=0$

I'm trying to solve the following Differential Equation: $\left( 1-2\lambda\frac{\partial}{\partial z}\right)w(x,y,z)-g(x,y,z+h)+2 \lambda h(x,y,z)=0$ The unknown function is $w(x,y,z)$. The ...
0
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1answer
33 views

Proving integration formulas from scratch

Prove the following integration formulas from scratch? (I uploaded them)
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1answer
41 views

Spectral Measures: Riemann-Lebesgue

Given a Hilbert space $\mathcal{H}$ and let the Lebesgue measure be $\lambda$. Consider a Borel spectral measure $E:\mathcal{B}(\mathbb{C})\to\mathcal{B}(\mathcal{H})$. Denote its associated ...
0
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1answer
38 views

How to calculate “general” integral $\int\limits_{a}^{b}f(x)^2dx$?

How to calculate "general" integral: $\int\limits_{a}^{b}f(x)^2dx$?
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votes
4answers
120 views

Integral: $\int_0^{\pi/12} \ln(\tan x)\,dx$

I am trying to evaluate: $$\int_0^{\pi/12} \ln(\tan x)\,dx$$ I think the integral is quite simple but I am having a hard time evaluating it. I started with the result: $$\int_0^{\pi/4} \ln(\tan ...
0
votes
1answer
37 views

What is this integration “method” name?

I see that people often write this equality: $$\int\limits_a^bf(x)\,\mathrm dx=\int\limits_{f(a)}^{f(b)}f(x)\,\mathrm df(x)$$ when dealing with functins in general, that is when something is trying ...
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3answers
56 views

The value of $\int_0^{2\pi}\cos^{2n}(x)$ and its limit as $n\to\infty$

Calculate $I_{n}=\int\limits_{0}^{2\pi} \cos^{2n}(x)\,{\rm d}x$ and show that $\lim_{n\rightarrow \infty} I_{n}=0$ Should I separate $\cos^{2n}$ or I should try express it in Fourier series?
7
votes
2answers
139 views

Evaluating $\int_0^1 \frac{t^{a-1}}{1-t}-\frac{ct^{b-1}}{1-t^c}\ dt$

At first sight it looks like the integral below $$\int_0^1 \frac{t^{a-1}}{1-t}-\frac{ct^{b-1}}{1-t^c}\ dt$$ can be evaluated by using some geometric series. What else can we do? Is there a fast easy ...
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0answers
12 views

Solving ODE involving matrices

We have a given ODE $ K(x)_{_{3 \times 3}}=xC_1K(x)+x^3C_2K'(x) \tag 1$ where $C_1,C_2$ are constant skew symmetric matrices of dimension $3 \times 3$ with determinant $0$. How do we solve ...
1
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1answer
16 views

how to evaluate $\int \left(\hat{V}\times \frac{d^2\hat{V}}{dt^2}\right)dt$?

if $\hat{V}\left(t\right)$ is a vector function of $t$, find the indefinite integral $\int \left(\hat{V}\times \frac{d^2\hat{V}}{dt^2}\right)dt$ To solve thi first i find for the integrand with ...
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0answers
20 views

CDF of ratio of Gamma distribution with different parameters

Let $X$ be gamma distributed random variable with parameters $a$ and $b$. Let $W$ be gamma distributed random variable with parameters $c$ and $d$, such that \begin{equation} f_X(x) = ...
1
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4answers
36 views

solve indefinite integral

I have this indefinite integral $\int 3 \sqrt{x}\,dx$ to solve. My attempt: $$\int 3 \sqrt{x}\,dx = 3 \cdot \frac {x^{\frac {1}{2} + \frac {2}{2}}}{\frac {1}{2} + \frac {2}{2}}$$ $$\int 3 ...
0
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0answers
24 views

Understanding integration and substitution

I'm an undergraduate student in EE. I often see that when talking about voltages, curents ... being expressed like functions of some independed variable (time) and when calculating integrals people ...
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2answers
30 views

Integral involving radicals

Can anyone give some hint as to how to proceed in solving this integral: $$ \int \frac{u}{\sqrt{R^{2}+r^{2}-2Rru}}\mathrm{d}u $$
2
votes
1answer
19 views

Limit integration

I have: $$ F(a)=\int_0^a(x^2+1)e^{-x/2} dx $$ I have done the integration: $$ \int(x^2+1)e^{-x/2}=-2e^{-x/2}(x^2+4x+9)+C$$ What is (if possible): $$ lim_{a \to \infty} F(a)$$ I tried: $$ ...
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0answers
22 views

Jacobian determinant of unitary transformation

Is the Jacobian determinant of a unitary transformation equal to one? I ask because I get that impression from the appendix of this paper. They have spherical coordinates for two particles, ...
0
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0answers
21 views

i am trying to interpret this function I think it is a volume for double integral

The problem is to understand the integration area for : $ \int(x,y)dy$ from 0 to sqrt(4-x^2) multiplied by integral $\int dx $ from -1 to - sqrt(3) so is this saying that the area of integration ...
8
votes
2answers
509 views

Integral of rational function with higher degree in numerator

How do I integrate this fraction: $$\int\frac{x^3+2x^2+x-7}{x^2+x-2} dx$$ I did try the partial fraction decomposition: $$\frac{x^3+2x^2+x-7}{x^2+x-2} = \frac{x^3+2x^2+x-7}{(x-1)(x+2)}$$ And: ...
0
votes
2answers
34 views

Complex integration along a curve

I have to calculate this integral: $$ \int_C e^z\,dz $$ where $C$ is the circle $|z - jπ/2| = π/2$ from the point $z = 0$ to the point $z = jπ$. I know how to calculate these with circles which ...
1
vote
1answer
30 views

Solution to differential equation $\left( 1-\lambda\frac{\partial}{\partial z}\right)w(x,y,z)-g(x,y,z+h)=0$

I'm trying to solve the following differential equation: $\left( 1-\lambda\frac{\partial}{\partial z}\right)w(x,y,z)-g(x,y,z+h)=0$ here $g(x,y,z+h)$ is a known function that however i will leave ...
0
votes
0answers
20 views

Inverse fourier transform using Parseval relation.

Please, if someone can help with this question I will be grateful. Considering Parseval's relation, show that the Inverse Fourier Transform can be written as $$ f(t)=\int_{-\infty}^\infty ...
1
vote
0answers
49 views

Further integrals of the seemingly unintegrable

I remember having to solve the following problem: Let \begin{equation} I_n=\int_0^{1}\frac{x^ndx}{\sqrt{x^3+1}}. \end{equation} Prove \begin{equation} ...