- #1
phantomcow2
- 52
- 0
Trig substitition--lost in identities
[tex]\int[/tex][tex]\frac{dx}{x^{2}\sqrt{x^{2}+1}}[/tex]
It's pretty obvious that this is a trig substitution problem requiring use of tangent.
[tex]x=tan\theta[/tex]
[tex]dx=sec^{2}\theta[/tex]
[tex]x^{2}=tan^{2}\theta[/tex]
Substitute it in.
[tex]\frac{sec^{2}\theta}{tan^{2}\theta\sqrt{tan^{2}\theta + 1}}[/tex]
But the [tex]\sqrt{tan^{2}\theta + 1}[/tex] simplifies to [tex]sec\theta[/tex]
Now before I integrate, I need to simplify. The obvious simplification is the [tex]sec^{2}\theta[/tex] and [tex]sec\theta[/tex] in the denominator, leaving me with
[tex]\frac{sec\theta}{tan^{2}\theta}[/tex]
This is starting to look fishy to me. I think I've begun to develop an instinct telling me when I am doing something incorrectly. I simplify this to [tex]\frac{cos\theta}{sin^{2}\theta}[/tex]
Now I need to ingrate this, but it doesn't look promising. Can someone tell me where I went wrong?
Homework Statement
[tex]\int[/tex][tex]\frac{dx}{x^{2}\sqrt{x^{2}+1}}[/tex]
Homework Equations
It's pretty obvious that this is a trig substitution problem requiring use of tangent.
The Attempt at a Solution
[tex]x=tan\theta[/tex]
[tex]dx=sec^{2}\theta[/tex]
[tex]x^{2}=tan^{2}\theta[/tex]
Substitute it in.
[tex]\frac{sec^{2}\theta}{tan^{2}\theta\sqrt{tan^{2}\theta + 1}}[/tex]
But the [tex]\sqrt{tan^{2}\theta + 1}[/tex] simplifies to [tex]sec\theta[/tex]
Now before I integrate, I need to simplify. The obvious simplification is the [tex]sec^{2}\theta[/tex] and [tex]sec\theta[/tex] in the denominator, leaving me with
[tex]\frac{sec\theta}{tan^{2}\theta}[/tex]
This is starting to look fishy to me. I think I've begun to develop an instinct telling me when I am doing something incorrectly. I simplify this to [tex]\frac{cos\theta}{sin^{2}\theta}[/tex]
Now I need to ingrate this, but it doesn't look promising. Can someone tell me where I went wrong?