Thursday, February 25, 2016

int 1 / ((x-1)sqrt(x^2-2x)) dx Find or evaluate the integral by completing the square

We have to evaluate the integral: \int \frac{1}{(x-1)\sqrt{x^2-2x}}dx
We can write the integral as:
\int \frac{1}{(x-1)\sqrt{x^2-2x}}dx=\int \frac{1}{(x-1)\sqrt{(x-1)^2-1}}dx
  Let x-1=t
So , dx=dt
hence we can write,
\int \frac{1}{(x-1)\sqrt{(x-1)^2-1}}dx=\int \frac{1}{t\sqrt{t^2-1}}dt
 Let u=t^2
So, du=2tdt
implies, dt=\frac{1}{2t}du
Therefore we have,
\int\frac{1}{t\sqrt{t^2-1}}dt=\int \frac{1}{t\sqrt{u-1}}.\frac{du}{2t}
                  =\int \frac{1}{2u\sqrt{u-1}}du
Now let v=\sqrt{u-1}
So, dv=\frac{1}{2\sqrt{u-1}}du=\frac{1}{2v}du
Hence we have,
\int \frac{1}{2u\sqrt{u-1}}du=\int \frac{2vdv}{2(v^2+1)v}
                   =\int \frac{dv}{v^2+1}
                    =tan^{-1}(v)+C  where C is a constant.
                     =tan^{-1}(\sqrt{u-1})+C
                      =tan^{-1}(\sqrt{t^2-1})+C
                      =tan^{-1}(\sqrt{(x-1)^2-1})+C
                       =tan^{-1}(\sqrt{x^2-2x})+C
 

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