Convergence Divergence Help

  • Thread starter cooltee13
  • Start date
  • #1
cooltee13
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Homework Statement


Well I am studying for my final which is in a couple of days, and I am stuck on this topic of convergence of improper integrals. I've been doing a couple and I thought I was doing them correctly but my answers do not go with the answers I was given. So I am stressing out over it. Here are the questions I have:

Determine whether the following integrals converge or diverge, evaluate those that converge.

1) Integral from -inf to 1 of: dx/(x-2)^5

2) Integral from 0 to inf of: e^x dx


Homework Equations


Just the standard limit and integral equations


The Attempt at a Solution


1) I split the integral into two pieces, a) the int from -inf to 0: 1/(x-2)^5 and b) the int from 0 to 1: 1/(x-2)^5 I then took the limit of each integral, so I got:
a) lim as t approaches -inf of a. solving the limit I got (1/-2 - 1/inf) which = -1/2 and is convergant.

b) lim as t approaches 0 of b. solving that limit I came up with a divergent answer. making the original Integral divergent. however the answer I was given says that the integral is convergent. I am confused on this.
-------------------------------------------------------------------------
2) integral from 0 to inf of e^x dx
I split this integral into two pieces as well. a) integral from 0 to 1: e^x dx and b) integral from 1 to inf: e^x dx

Solving each part, I got two convergent answers, but the answer said that its divergent. I am wrong again :yuck:.

Please help me out, and explain why. I have the answers, that's not what I am going for..I just want to understand this so I can do well on my final. Thanks guys
 

Answers and Replies

  • #2
rootX
465
4
I didn't get why you are splitting things.
You only do that when you have a discontinuity like going from -1 to 1 for 1/x.

1/(x-2)^2 .. gives -1/(x-2) ] from t to 1, and take limit of t as it appraches -inf

same thing for e^x
 
  • #3
rootX
465
4
b) the int from 0 to 1: 1/(x-2)^5 I then took the limit of each integral, so I got: ...

also, you did this step wrong!
you don't have any inf/discountinity from 0 to 1
 

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