- #1
lendav_rott
- 232
- 10
As a way of practicing my Latex skills and asking about antiderivatives.
Suppose we have
[tex]\int\frac{1}{x^2 + a}dx
[/tex]
a is just a constant
Now I recognize this to be
[tex]\int\frac{dx}{x}[/tex] whose antiderivative is [tex]\frac{\ln{|x|}}{dx} + C[/tex]
The question is: In the first function, I could make a trigonometric substition - but do I have to ?
Is the antiderivative [tex]\frac{ln|x^2 + a|}{2x} + C[/tex] incorrect? The logic being if I only found the derivative of ln|x2+ a| I would have [tex]\frac{1}{x^2 + a} * \frac{d}{dx}(x^2+a)[/tex] so to cancel the 2nd factor I would divide the antiderivative by it.
If a=1 I could rewrite the 1st integral as
[tex]\int\frac{dx}{\tan^2x +1} = \int\frac{dx}{\sec^2x} = \int\cos^2xdx = \frac{1}{2}[ \sin(x) \cos(x) + x] + C[/tex] assuming i did my math correctly with this one.
but can it also be [tex]\frac{ln(x^2 + 1)}{2x} + C[/tex]also on a side note, if there is an expression that is clearly Always positive such as x2 + 1 , do I have to use the abs value signs when I take the natural log of it?
Cheers
Suppose we have
[tex]\int\frac{1}{x^2 + a}dx
[/tex]
a is just a constant
Now I recognize this to be
[tex]\int\frac{dx}{x}[/tex] whose antiderivative is [tex]\frac{\ln{|x|}}{dx} + C[/tex]
The question is: In the first function, I could make a trigonometric substition - but do I have to ?
Is the antiderivative [tex]\frac{ln|x^2 + a|}{2x} + C[/tex] incorrect? The logic being if I only found the derivative of ln|x2+ a| I would have [tex]\frac{1}{x^2 + a} * \frac{d}{dx}(x^2+a)[/tex] so to cancel the 2nd factor I would divide the antiderivative by it.
If a=1 I could rewrite the 1st integral as
[tex]\int\frac{dx}{\tan^2x +1} = \int\frac{dx}{\sec^2x} = \int\cos^2xdx = \frac{1}{2}[ \sin(x) \cos(x) + x] + C[/tex] assuming i did my math correctly with this one.
but can it also be [tex]\frac{ln(x^2 + 1)}{2x} + C[/tex]also on a side note, if there is an expression that is clearly Always positive such as x2 + 1 , do I have to use the abs value signs when I take the natural log of it?
Cheers
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