Trouble with tables

  • Thread starter Jbreezy
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  • #1
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Homework Statement



Hi I have a broad quesiton. I'm trying to make something fit the form of something in one of the tables in my book I can't seem to quite make it.
integral (1/2x^3 - 3x^2) dx


Homework Equations





The Attempt at a Solution



I just messed with the denomator.
1/(x^2(2x-3))

I can't find something in the tables that fits this. Does anyone have any ideas?
 

Answers and Replies

  • #2
3,812
92

Homework Statement



Hi I have a broad quesiton. I'm trying to make something fit the form of something in one of the tables in my book I can't seem to quite make it.
integral (1/2x^3 - 3x^2) dx


Homework Equations





The Attempt at a Solution



I just messed with the denomator.
1/(x^2(2x-3))

I can't find something in the tables that fits this. Does anyone have any ideas?
You can write the given integral as:
$$\int \cfrac{dx}{x^2\cdot x\left(2-\cfrac{3}{x}\right)}$$
A substitution would make it very easy. It is easy to spot.
 
  • #3
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I'm sorry I still don't see it. Sub for u = x^2?
 
  • #4
3,812
92
I'm sorry I still don't see it. Sub for u = x^2?
You have x^2 in the denominator i.e 1/x^2. This is the derivative of something very familiar. :rolleyes:
 
  • #5
34,667
6,379

Homework Statement



Hi I have a broad quesiton. I'm trying to make something fit the form of something in one of the tables in my book I can't seem to quite make it.
integral (1/2x^3 - 3x^2) dx
What you wrote is this:
$$ \int \frac 1 2 x^3 - 3x^2 dx$$

To indicate that 2x3 - 3x2 is in the denominator, put parentheses around the denominator, not the whole fraction, like this 1/(2x3 - 3x2).
 
  • #6
Ray Vickson
Science Advisor
Homework Helper
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Homework Statement



Hi I have a broad quesiton. I'm trying to make something fit the form of something in one of the tables in my book I can't seem to quite make it.
integral (1/2x^3 - 3x^2) dx


Homework Equations





The Attempt at a Solution



I just messed with the denomator.
1/(x^2(2x-3))

I can't find something in the tables that fits this. Does anyone have any ideas?
Partial fractions.
 
  • #7
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0
You have x^2 in the denominator i.e 1/x^2. This is the derivative of something very familiar. :rolleyes:
No sure. This would be -1/x whose derivavite is is 1/x^2 are you thinking ln of something? Ray says partial fractions but this is the section where you have to look them up in tables.
 
  • #8
3,812
92
No sure. This would be -1/x whose derivavite is is 1/x^2 are you thinking ln of something? Ray says partial fractions but this is the section where you have to look them up in tables.
Yes, use the substitution 1/x=t.

Partial fractions can also be used.
 

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