We will use integration by parts
int u dv=uv-int v du
We will need to apply integration by parts two times in order to eliminate x^2 from under the integral.
int_0^infty x^2e^-x dx=|[u=x^2,dv=e^-x dx],[du=2x dx, v=-e^-x]|=
-x^2e^-x+2int_0^infty xe^-x dx=|[u=x,dv=e^-x dx],[du=dx,v=-e^-x]|=
(-x^2e^-x-2xe^-x)|_0^infty+2int_0^infty e^-x dx=
(-x^2e^-x-2xe^-x-2e^-x)|_0^infty=
lim_(x to infty)(-x^2e^-x-2xe^-x-2e^-x)-(-0-0-2)=
To calculate the above limit we will use L'Hospital's rule:
lim_(x to a) (f(x))/(g(x))=lim_(x to a)(f'(x))/(f'(x))
First we rewrite the limit to fit the form of L'Hospital's rule.
lim_(x to infty) -x^2e^-x=lim_(x to infty)-x^2/e^x=
Now we differentiate.
lim_(x to infty)-(2x)/e^x=
This integral still yields infty/infty so we differentiate again. Notice that this corresponds to the second term in our calculation of the integral.
lim_(x to infty)-2/e^x=0
Now we know everything needed to calculate the integral.
-0-0-0+0+0+2=2
As we can see the integral converges to 2.
The image below shows the graph of the function and area under it corresponding to the integral. We can see that the function converges to zero "very fast" (there's hardly any area under the graph after 12). This implies likely convergence of the integral.
Monday, March 23, 2015
Calculus of a Single Variable, Chapter 8, 8.8, Section 8.8, Problem 23
Subscribe to:
Post Comments (Atom)
Why is the fact that the Americans are helping the Russians important?
In the late author Tom Clancy’s first novel, The Hunt for Red October, the assistance rendered to the Russians by the United States is impor...
-
The poem contrasts the nighttime, imaginative world of a child with his daytime, prosaic world. In the first stanza, the child, on going to ...
-
There are a plethora of rules that Jonas and the other citizens must follow. Again, page numbers will vary given the edition of the book tha...
-
Robinson Crusoe, written by Daniel Defoe, is a novel. A novel is a genre defined as a long imaginative work of literature written in prose. ...
-
Hello! This expression is already a sum of two numbers, sin(32) and sin(54). Probably you want or express it as a product, or as an expressi...
-
The title of the book refers to its main character, Mersault. Only a very naive reader could consider that the stranger or the foreigner (an...
-
"The Wife's Story" by Ursula Le Guin presents a compelling tale that is not what it initially seems. The reader begins the sto...
-
In Celie's tenth letter to God, she describes seeing her daughter in a store with a woman. She had not seen her daughter since the night...
No comments:
Post a Comment