Integration by trigonometric substitution

cesaruelas
Messages
51
Reaction score
0

Homework Statement



∫ 1/(x√(25-x^2)) dx

Homework Equations



by trigonometic substitution I got the following equations
x = 5 sin θ
dx = 5 cos θ dθ
√(25-x^2) = 5 cos θ
θ = sin^-1 (x/5)

The Attempt at a Solution



I solved it and got the following (so far, it is okay with wolfram alpha):

∫ 1/(x√(25-x^2)) dx = (1/5) ∫ csc θ dθ

according to me, the result should be (1/5) ( ln ( 5/x - √(25-x^2)/x ) )
but according to wolfram alpha it should be (1/5) ( ln (x) - ln ( √(25-x^2) + 5 ) )

are these two results equivalent?

 
Physics news on Phys.org
Differentiate the answers and see if you get the integrand. That will tell you.
 
\displaystyle \ln (x) - \ln ( \sqrt{25-x^2} + 5 )=\ln\left(\frac{x}{\sqrt{25-x^2} + 5}\right)
\displaystyle =-\ln\left(\frac{\sqrt{25-x^2}}{x}+\frac{5}{x}\right)​

So, there's only a disagreement about the sign.
 
There are two things I don't understand about this problem. First, when finding the nth root of a number, there should in theory be n solutions. However, the formula produces n+1 roots. Here is how. The first root is simply ##\left(r\right)^{\left(\frac{1}{n}\right)}##. Then you multiply this first root by n additional expressions given by the formula, as you go through k=0,1,...n-1. So you end up with n+1 roots, which cannot be correct. Let me illustrate what I mean. For this...

Similar threads

Replies
11
Views
2K
Replies
6
Views
1K
Replies
27
Views
2K
Replies
12
Views
2K
Replies
6
Views
2K
Replies
10
Views
11K
Back
Top