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Thanks. I'm not really sure that I ''needed'' to use some sort of comparison to show it didn't converge, as it did integrate nicely, but its good to have a second opinion, be that mathematical or otherwise. --[[User:Jmason|John Mason]] | Thanks. I'm not really sure that I ''needed'' to use some sort of comparison to show it didn't converge, as it did integrate nicely, but its good to have a second opinion, be that mathematical or otherwise. --[[User:Jmason|John Mason]] | ||
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| + | It does not converge for me either, but Josh, be careful with your comparison. It's not really valid to use <math>1/\sqrt{x^4}</math> as the function for comparison, because <math>x/\sqrt{x^4} < x/\sqrt{x^4-1}</math>, albeit infinitesimally less. I used <math>(g(x) = 1/x^{0.99}) > (f(x) = x/\sqrt{x^4-1}) </math>, in which case P < 1 and diverges. --[[User:Reckman|Randy Eckman]] 15:44, 26 October 2008 (UTC) | ||
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| + | That is true. But if you start out with that as your comparison: | ||
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| + | <math>\sqrt{x^4} > \sqrt{x^4 - 1} </math> | ||
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| + | <math>\text{For } x > 0 </math> | ||
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| + | <math>x\sqrt{x^4} > x\sqrt{x^4 - 1} </math> | ||
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| + | <math>\frac{x}{\sqrt{x^4 - 1}} > \frac{x}{\sqrt{x^4}} = \frac{x}{x^2} = \frac{1}{x} </math> | ||
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| + | <math>\int^\infty_4 \frac{dx}{x} = \infty </math> | ||
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| + | Certainly much cleaner than working with decimal powers. --[[User:Jmason|John Mason]] | ||
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| + | Also, remember, <math>\int_1^{\infty}\frac{1}{x^p}dx</math> diverges as long as <math>p\le 1</math>. So if p = 1, it still diverges. That's why it's okay to use the comparison I used. I did what John did, only I multiplied the inequality by x after taking the inverse of both sides and switching the inequality sign.[[User:Jhunsber|His Awesomeness, Josh Hunsberger]] | ||
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| + | == Pg. 329, #16 == | ||
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| + | How accurate do we need to make our answers for the roots? After four iterations, I have the first point accurate to four digits. The text doesn't specify a number of correct digits, and out of curiosity I found the precise roots on Mathematica. I don't know how the textbook could expect us to calculate exactly this: | ||
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| + | [[Image:Mathematicapg329num16_MA181Fall2008bell.PNG]] | ||
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| + | {x -> 0.630115} | ||
| + | {x -> 2.57327} | ||
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| + | --[[User:Reckman|Randy Eckman]] 17:43, 26 October 2008 (UTC) | ||
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| + | I went to 10 digits, as that was all my calculator could show. And for the record "Reckman" is a very cool name. --[[User:Jmason|John Mason]] | ||
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| + | And I went to five digits because that's all my calculator would show me (I think I can change that, but i wasn't sure how). [[User:Jhunsber|His Awesomeness, Josh Hunsberger]] | ||
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| + | == Pg.329, #5 == | ||
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| + | I don't really understand how to do #5. It seems like there isn't an actual function. Are we supposed to use maple? Can someone help get me started? | ||
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| + | --[[User:Klosekam|Klosekam]] 16:19, 27 October 2008 (UTC) | ||
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| + | Since you need to know where | ||
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| + | <math>e^{-x} = 2x + 1</math> | ||
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| + | you can just subtract out one side and solve for the roots (aka at what value of x the function takes zero). So you can use | ||
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| + | <math>f(x) = 2x + 1 -e^{-x}</math> | ||
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| + | --[[User:Jmason|John Mason]] | ||
Latest revision as of 08:53, 28 October 2008
8.8 #54
Does this indefinite integral converge for anyone? Also, if you are having trouble with the integral, take a look at the derivatives of inverse hyperbolic functions. --John Mason
It does not converge for me. I used direct comparison to test whether it converges or not. I started by comparing $ \sqrt{x^4-1} $ and $ \sqrt{x^4} $ It was easy from there. --Josh Visigothsandwich
Thanks. I'm not really sure that I needed to use some sort of comparison to show it didn't converge, as it did integrate nicely, but its good to have a second opinion, be that mathematical or otherwise. --John Mason
It does not converge for me either, but Josh, be careful with your comparison. It's not really valid to use $ 1/\sqrt{x^4} $ as the function for comparison, because $ x/\sqrt{x^4} < x/\sqrt{x^4-1} $, albeit infinitesimally less. I used $ (g(x) = 1/x^{0.99}) > (f(x) = x/\sqrt{x^4-1}) $, in which case P < 1 and diverges. --Randy Eckman 15:44, 26 October 2008 (UTC)
That is true. But if you start out with that as your comparison:
$ \sqrt{x^4} > \sqrt{x^4 - 1} $
$ \text{For } x > 0 $
$ x\sqrt{x^4} > x\sqrt{x^4 - 1} $
$ \frac{x}{\sqrt{x^4 - 1}} > \frac{x}{\sqrt{x^4}} = \frac{x}{x^2} = \frac{1}{x} $
$ \int^\infty_4 \frac{dx}{x} = \infty $
Certainly much cleaner than working with decimal powers. --John Mason
Also, remember, $ \int_1^{\infty}\frac{1}{x^p}dx $ diverges as long as $ p\le 1 $. So if p = 1, it still diverges. That's why it's okay to use the comparison I used. I did what John did, only I multiplied the inequality by x after taking the inverse of both sides and switching the inequality sign.His Awesomeness, Josh Hunsberger
Pg. 329, #16
How accurate do we need to make our answers for the roots? After four iterations, I have the first point accurate to four digits. The text doesn't specify a number of correct digits, and out of curiosity I found the precise roots on Mathematica. I don't know how the textbook could expect us to calculate exactly this:
{x -> 0.630115} {x -> 2.57327}
--Randy Eckman 17:43, 26 October 2008 (UTC)
I went to 10 digits, as that was all my calculator could show. And for the record "Reckman" is a very cool name. --John Mason
And I went to five digits because that's all my calculator would show me (I think I can change that, but i wasn't sure how). His Awesomeness, Josh Hunsberger
Pg.329, #5
I don't really understand how to do #5. It seems like there isn't an actual function. Are we supposed to use maple? Can someone help get me started?
--Klosekam 16:19, 27 October 2008 (UTC)
Since you need to know where
$ e^{-x} = 2x + 1 $
you can just subtract out one side and solve for the roots (aka at what value of x the function takes zero). So you can use
$ f(x) = 2x + 1 -e^{-x} $
