Concerns all aspects of integration, including the integral definition and computational methods. For questions solely about the properties of integrals, use in conjunction with (indefinite-integral), (definite-integral), (improper-integrals) or another tag(s) that typically describe(s) the types of ...

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0answers
9 views

Asymptotic behaviour of an integral depending on a parameter

I am trying to compute the asymptotics on $t$ of the following integral: \begin{equation} I(t)=\int_{\mathbb{R}^{n}}e^{-|\lambda|^{2}/2t}\prod_{i<j}\left( e^{\lambda_{j}/t}-e^{\lambda_{i}/t} ...
1
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7answers
80 views

Integrate $\int \frac{x\cos x}{\sin^2x}dx$

$$\int \frac{x\cos x}{\sin^2x}dx$$ $$\int \frac{x\cos x}{\sin^2x}dx=\int \frac{x\cos x}{1-\cos^2x}dx=\int \frac{x\cos x}{(1-\cos x)(1+\cos x)}dx$$ How can I find the two fractions? if there are ...
2
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0answers
18 views

What would be simple way of calculating the area of the visible parts of a 3D PieChart's slice?

I have created a 3D Pie Chart able to be rotated. -> http://plnkr.co/edit/QIYu8sJUWPmxcby1ky9l?p=preview I did it to demonstrate how the visual perception of data in a Pie Chart can be distorted ...
-1
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1answer
15 views

how to prove if $f$ is integrable and $f'$ too then the limit of $f$ is zero when $x$ go to infinity?

If $f$ is a real function on $\mathbb R$ and we have $\int_1^\infty |f(x)|dx < \infty$ and $\int_1^\infty |f'(x)|dx < \infty$ then $\lim_{x\to\infty}f(x)=0$ ?
1
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0answers
31 views

Compute $\frac{d}{dt}\int_0^t e^{x(s)}ds$, where $x$ is a standard Brownian motion.

How to compute the following differentiation? Is there a general rule that can be applied? $$\frac{d}{dt}\int_0^t e^{x(s)}ds$$ in the case of $x=W$ where $W$ is a standard brownian motion, is there ...
1
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2answers
54 views

Evaluate $\int_{-\infty}^{\infty}x^2 e^{-\alpha x^2+\beta x}dx$

As a consequence of this Q, I need some help evaluating the following integral: $$\int_{-\infty}^{\infty}x^2 e^{-\alpha x^2+\beta x}dx$$ Integration by parts wouldn't simplify things and I guess that ...
0
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1answer
30 views

finding area using iterated integral

I am trying to find the area enclosed between $f(x)=\sin x$ and $g(x)=\cos x$ between $x= \pi/4$ to $x = 5 \pi/4$. I got $\int_{\pi/4}^{5\pi/5}\int_{\cos x}^{ \sin x} dydx$. But I am not getting the ...
4
votes
1answer
31 views

How would you integrate this?

If we had the following integral: $$\int_{a}^{b} {\big(1+x^2 \big)^s} \space dx$$ Where $s$ is not given. Is there any general formula for this integration that works for all $s\in \mathbb{R}$?
1
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3answers
17 views

Proof of integral involving the inverse hyperbolic secant and cosent

We know that $$ \int \frac{dx}{x \sqrt{a^2 \pm x^2} } = -\frac{1}{a} \ln \frac{a+ \sqrt{a^2 \pm x^2}}{\lvert x\rvert }+C$$ I tried proving this integral setting $x = a \ \mathrm{csch} \ u $ and using ...
2
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2answers
32 views

Advanced calculus, Riemann integral.

If $f$ is (Riemann) integrable on $[a,b]$ and if $\int_{a}^{b} fh=0$ for all continuous function $h$, then $f(x)=0$ for all points of continuity of $f$. I know if we have $f$ being continuous on ...
5
votes
1answer
67 views

Improper integral: $\int_1^\infty\frac{\sin(\sqrt{x})}{\sqrt{x}}dx $.

mathematica is reporting that the improper integral $\int_1^\infty\frac{\sin(\sqrt{x})}{\sqrt{x}}dx $ coverges to $2\cos(1)$. However, when I try to confirm this by actually integrating it using ...
0
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1answer
24 views

Proof of integral involving hyperbolic tangent

We know that $$ \int \frac{dx}{a^2-x^2} = \frac{1}{2a}\ln \left| \frac{a+x}{a-x}\right| +C$$ (That absolute value sign is supposed to be longer. I apologize for ignorance on how to make that longer on ...
1
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0answers
19 views

Flux Through a Closed Curve - Orientation

I want to compute $$\int_{C}\boldsymbol{F}\cdot\boldsymbol{n}\, ds\qquad\quad \boldsymbol{F}=\langle x, y^2\rangle$$ where $C$ is the curve given by the triangle with vertices $(-1,0)$, $(0,1)$ and ...
2
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0answers
17 views

Changing integration bounds

I came accross this line: $$x\in [0,1],y\in [0,1]$$ $$E(Y|x)=\int_y y*g(y|x)dx=\int _0^1y*g(y|x)dy$$ Can someone please explain how the second equality holds! Thanks.
3
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1answer
25 views

Trigonometric integrals and limits

Show $$\lim_{N\to\infty}g_N(\theta_N)=2\int^\pi_0\frac{\sin x}{x}dx-\pi,$$ where $$g_N(\theta_N)=\int_0^{\theta_N}\frac{\sin[(N+1/2)x]}{\sin(x/2)}dx-\pi,$$ $$\theta_N=\frac{\pi}{N+1/2},$$ and ...
3
votes
2answers
42 views

What is the the integral of $\sqrt{x^a + b}$?

How do you evaluate $\displaystyle\int\sqrt{x^a + b}\,\,\text{dx}$, where $a \neq 0$ and $a \neq 1$? For example, how do you evaluate $\displaystyle\int\sqrt{x^2 + 1}\,\text{dx}$? If we let ...
4
votes
0answers
35 views

Integral involving power of trigonometric functions

I'm having a technical problem evaluating the following integral: $$\int_{r=0}^1\int_{\theta=0}^{\pi \over2} \cos^{2\epsilon -1}\theta \sin^{\epsilon-1}\theta e^{-ikr\sin^\epsilon\theta}d\theta dr$$ ...
0
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1answer
50 views

Regarding Apostol's theory of integration

I have some questions regarding the theory of integration as discussed in Tom Apostol's Calculus. Integration is defined using step functions. My question is, is this definition he presents equivalent ...
1
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3answers
58 views

integrate $\int \sin^{4}x\cos^{2}x$ [duplicate]

$$\int \sin^{4}x\cos^{2}xdx$$ $$\int \sin^{4}x\cos^{2}xdx=\int (\sin x \cos x)^{2}\sin^2xdx=\int \left(\frac{\sin^{2}2x}{2}\right)\left(\frac{1}{2}-\frac{\cos2x}{2}\right)dx=\int ...
1
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6answers
99 views

Integrate $\int_{-\infty}^\infty xe^{-\alpha x^2+\beta x}dx$ [duplicate]

I am familiar with the gauusian integral $$\int_{-\infty}^\infty e^{-\alpha x^2+\beta x}dx=\sqrt{\frac{\pi}{\alpha}}e^{\beta^2/(4\alpha)}$$ Could anyone help me to find out the value of the following? ...
0
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1answer
32 views

Limit with number of integrals tending to infinity

Let $F_0(x) = \ln x$. For $n \geq 0$ and $x >0$, let $F_{n+1}(x)=\int_0^x F_n(t)dt$. Evaluate $$\lim_{n \to \infty} \frac{n! F_n(1)}{\ln n}$$ Because the final intergal is from $0$ to $1$, I ...
1
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1answer
46 views

To prove or refute: $\lim_{N \to \infty} \frac{1}{N} \sum_{n=1}^{N} f\left( \frac{n}{N} \right) = 1$ then $f \in R\left( \left[ 0, 1 \right] \right)$ [on hold]

Let $f : \left[ 0, 1 \right] \to \mathbb{R}$ such that $$\lim_{N \to \infty} \frac{1}{N} \sum_{n=1}^{N} f\left( \frac{n}{N} \right) = 1.$$ Then, $f \in R\left( \left[ 0, 1 \right] \right)$ and ...
2
votes
1answer
53 views

Integral of $\int_0^{2\pi}{e^{({x\sin\theta)}}\ \text{d}\theta}$

I would like to compute $$f(x)= \int_0^{2\pi}{e^{({x\sin\theta)}}\ \text{d}\theta}$$ I already know that it equals $2\pi\sum_{j\geq0}{(\frac{x^j}{2^jj!})^2}$. I'm already happy since it provides an ...
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1answer
24 views

Evaluate a double integral over a region $R$

Let $R$ be the refion enclosed by $x^2+4y^2\ge 1$ and $x^2+y^2\le 1$. Calculate $$\iint_R \lvert xy\rvert\,dxdy$$ I think the answer is $0$ because the area in the positive quadrants cuts the ...
2
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3answers
64 views

Dealing with integrals of the form $\int{e^x(f(x)+f'(x))}dx$

Integrals of the form $$\int{e^x(f(x)+f'(x))}dx$$ are very common. And I have seen this form appearing in several exam papers.But the problem I face with this particular type of integral is finding ...
3
votes
1answer
90 views

integrate $\int \frac{dx}{4+3sin2x}$

$$\int \frac{dx}{4+3\sin (2x)}$$ $u=2x$ $du=2dx$ $$\frac{1}{2}\int \frac{du}{4+3\sin(u)}$$ $v=\tan(\frac{u}{2})$ $du=\frac{2dv}{1+v^2}$ \begin{align*} \frac{1}{2}\int ...
0
votes
0answers
23 views

Determination of a formula of an area of shape limited by curves using integrals?

I did two examples of determining an area limited by curves and I'm not sure if I did them right. I would really appreciate if someone checked my solution and fixed any possible mistakes. Ex.1 Area ...
1
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0answers
24 views

Discontinuous parametric integral function

Is there an example of a function $f:[0,1] \times [0,1] \to \mathbb{R}$ such that for all $x \in [0,1]$ the function $\phi(y) = f(x,y)$ is continuous in $y$ and for all $y \in [0,1]$ the function ...
1
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1answer
27 views

Calculation of area in 2 definite integrals given function $y=x^2$

Here is a graph for $y=x^2$ Given that the area in blue is equal to the area in pink, find a in terms of b and solve for a. My attempt: From the graph I can see that :$a^2=b$ and $a=\sqrt b$ ...
0
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1answer
16 views

Finding Intervals after Changing the Order of Integration

Problem: Let $f(x) = \int_0^x e^{t^2} \,dt.$ Find the average value of f on the interval $[0, 1]$. Thoughts: $\int_0^x e^{t^2} \,dt$ is a non-elementary integral. The average value of f, I believe, ...
0
votes
0answers
18 views

The line integral of the polar angles of points of the XY plane through a closed curve

Let me ask the following question. Let $XY\setminus \{(0,0)\}$ denote the 2D XY plane excluding the origin point. And let $\mathbb{R}$ be the set of all real numbers. Let a function $f:XY\setminus ...
0
votes
0answers
68 views

If $y(t) = t\left(1-\int_0^ty(x)\,dx\right)+4\int_0^tx\,y(x)\,dx,$ then $\int_0^{\pi/2} y(t)\,dt$ is equal to?

Leibniz rule or Laplace transform? Let $y(t)$ be a continuous function on $[0,\infty)$. If $$y(t) = t\left(1-4\int_0^ty(x)\,dx\right)+4\int_0^tx\,y(x)\,dx,$$ then $\int_0^{\pi/2} y(t)\,dt$ ...
0
votes
1answer
23 views

What would be a good cartesian equation to represent the shape of a wine glass?

I want to find the volume of a wine glass by using either the disk or shell method (solids of revolutions). The wine glass doesn't have to be of any particular dimensions, however it should roughly ...
3
votes
2answers
76 views

How to solve integrals where you can't factor a polynomial?

Hi there guys I don't know if the title of the question should be the one for this but the thing is that I'm trying to solve this integral $\int \frac {\frac 12-u^2}{2u^4-2u^2+1}$$du$ and I have this ...
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1answer
64 views

How to solve this definite integration [on hold]

$$ I = \int\limits_0^\pi \frac{d\theta}{\left[(\alpha - \beta \cos \theta)^2 + c \right]^2 + d^2} $$ Source. I am stuck to solve the integration.
4
votes
3answers
280 views

Why is the surface area of a sphere not given by this formula?

If we consider the equation of a circle: $$x^2+y^2=R^2$$ Then I propose that the volume of half of a sphere of radius $R$ is given by the summation of the circumferences of the circles between the ...
0
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3answers
45 views

How to calculate $f(x, y, z)$ given $d f = \frac{\partial f}{\partial x} dx + \frac{\partial f}{\partial y} dy +\frac{\partial f}{\partial z} dz$

On a manifold with local coordinates $(x_1, \ldots, x_n)$ I have a closed 1-form $\omega$ for which $d \omega = 0$ holds. This means There must be a function $f(x_1, \ldots x_n)$ for which $d f = ...
1
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2answers
71 views

How to compute $\int_0^2(1+4t^2+9t^4)^{1/2}\text{d}t$?

The original question was: find the length $\ell$ of the curve $\gamma$ given the parametric equations: $$x=t~~~~~ y=t^2~~~~~ z=t^3 $$ from $t=0$ to $t=2$
0
votes
0answers
28 views

Laplace Transform of $e^{t^3}$

I have to find the Laplace transform of $$e^{t^3} u(t)$$ and I know that $u(t)$ will just change the integral from negative infinity to positive infinity to $0$ to positive infinity, but I'm stuck ...
0
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0answers
14 views

Integral of dot product of unit vector

I am having trouble with the following integral. $$\int \left(\bar{A} \cdot \hat{ F\left(\lambda\right)}\right)^p\mathrm ds$$ Note that the right hand side of the dot product is normalised. Where: ...
1
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2answers
25 views

Constancy of an integral function

Fix some $\ell\in\mathbb{R}^+$. Say that $f:\mathbb{R}^2\to\mathbb{R}_{\geq0}$ and $\mu:\mathbb{R}\to\mathbb{R}^+$ are functions satisfying the following: $f$ and $\mu$ are continuous. $f$ is ...
1
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1answer
24 views

Convolution using Integration

Using integration, how would I solve f(t) convolve g(t) given that $$f(t)=u(t)-u(t-5)$$ and $$g(t)=2[u(t)-u(t-1)]$$ I know it should be $$\int_0^6 f(\tau) \ast g(t-\tau)~ d\tau = ...
1
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1answer
10 views

Volume Exponential Function

I should find the Volume received by rotating the region bounded by: $y = e^x $, $ y = 0 $,$ x = 0 $, $ x = 1 $ rotated around the x axis. I know how to find it by using the disc method but I could ...
1
vote
1answer
54 views

Integrating $\int^1_0 \dfrac{x^2e^{\arctan x}}{\sqrt{x^2+1}}$

This is a very hard integral that I am trying to solve. I’ve tried many substitutions, integration by parts, but I cannot evaluate this. Are there any other approaches I can take to solve this ...
2
votes
3answers
78 views

integrate $\int\frac{\sin x}{1+\sin^{2}x}dx$

$$\int\frac{\sin x}{1+\sin^{2}x}\mathrm {dx}$$ $$\int\frac{\sin x}{1+\sin^{2}x}\mathrm {dx}=\int\frac{\sin x}{2-\cos^{2}x}\mathrm {dx}$$ $u=\cos x$ $du=-\sin x dx$ $$-\int\frac{\mathrm ...
1
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3answers
21 views

Laplace transform for $-t\cos(2t)$

This Laplace transform exercise is giving me a headache. I was trying to use the definition of the Laplace transform but when I make the $u$ and $dv$ substitutions for the integration by parts I never ...
1
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1answer
24 views

Example of a non square-integrable martingale?

Are there (simple) examples of martingales which aren't square integrable?
0
votes
1answer
27 views

use L1-convergence to show integral convergence

Let $f\in L^1([0,1])$, $g_n$ a sequence of continuous functions that converges in $L^1$ to some $g\in L^1([0,1])$. Now my question is: Does $\int_0^1 f(t)e^{g_n(t)} dt$ converge to $\int_0^1 ...
2
votes
1answer
179 views

Calculate $I=\int_0^{1}\frac{1+x}{x^2+x+1}\log\left({\frac{x}{1-x}}\right)\,\mathrm dx$ without using complex analysis

Calculate $$I=\int_0^{1}\frac{1+x}{x^2+x+1}\log\left({\frac{x}{1-x}}\right)\,\mathrm dx$$ without using complex analysis. How to calculate without using the residue theorem? The correct answer ...
3
votes
1answer
39 views

Which inequalities are there with stochastic integration?

Which inequalities can I use with stochastic integration? For example, with the standard lebesgue integral we have $$\left|\int_\Omega f(x) dx\right| \le M |\Omega|$$ (where $M$ is the maximum of ...