- #1
PWiz
- 695
- 116
Just for fun, I tried this rather trivial problem, but I think I went wrong somewhere:
$$\int arcsec(x) \ dx$$
Let ##arcsec(x)=y## . Then ##x=sec \ y##, or ##y=arccos(\frac 1{x})##
So the problem becomes $$\int arccos(\frac 1 {x}) \ dx$$
Let ##\frac 1 {x} = cos \ u## , so that ##dx = secu \ tanu \ du##.
So the questions simplifies to ##\int u \ secu \ tanu \ du##. If I integrate by parts, then this equals to
$$u \int secu \ tanu \ du - \int (\int secu \ tanu \ du) du$$
Since ##\int secu \ tanu \ du = secu##, I get ##u \ secu - \int secu \ du##
##\int secu \ du = ln(\sqrt{\frac{1+sinu}{1-sinu}})##
Substituting the Xs, I get $$x \ arcsec(x) - ln(\sqrt{\frac{1+\sqrt{1-\frac{1}{x^2}}}{1-\sqrt{1-\frac{1}{x^2}}}}) + c= x \ arcsec(x) - ln(\sqrt{\frac{x+\sqrt{x^2-1}}{x-\sqrt{x^2-1}}}) + c $$
where ##c## is an arbitrary constant. I checked my answer with Wolfram Alpha here:
http://integrals.wolfram.com/index.jsp?expr=arcsec(x)&random=false
As you can clearly see, the answer is different from mine. In which step did I goof?
$$\int arcsec(x) \ dx$$
Let ##arcsec(x)=y## . Then ##x=sec \ y##, or ##y=arccos(\frac 1{x})##
So the problem becomes $$\int arccos(\frac 1 {x}) \ dx$$
Let ##\frac 1 {x} = cos \ u## , so that ##dx = secu \ tanu \ du##.
So the questions simplifies to ##\int u \ secu \ tanu \ du##. If I integrate by parts, then this equals to
$$u \int secu \ tanu \ du - \int (\int secu \ tanu \ du) du$$
Since ##\int secu \ tanu \ du = secu##, I get ##u \ secu - \int secu \ du##
##\int secu \ du = ln(\sqrt{\frac{1+sinu}{1-sinu}})##
Substituting the Xs, I get $$x \ arcsec(x) - ln(\sqrt{\frac{1+\sqrt{1-\frac{1}{x^2}}}{1-\sqrt{1-\frac{1}{x^2}}}}) + c= x \ arcsec(x) - ln(\sqrt{\frac{x+\sqrt{x^2-1}}{x-\sqrt{x^2-1}}}) + c $$
where ##c## is an arbitrary constant. I checked my answer with Wolfram Alpha here:
http://integrals.wolfram.com/index.jsp?expr=arcsec(x)&random=false
As you can clearly see, the answer is different from mine. In which step did I goof?