Thursday, 29 June 2017

A Difficult Integral

I'm wanting to keep up my LaTeX skills so I'm returning to an integral, specifically: 10xx3+1dx
The integral is not all that difficult really but it does involve a fair number of steps to get it right. Here is a graph of what is being attempted:


I used symbolab to work through the solution which turns out to be:19(3πln(8))
The first step in the solution is to factorise the denominator. This is easy because x3+1 is a sum of two cubes and so can be written as (x+1)(x2x+1). Using partial fractions, the integral can be converted into the following form:1310x+1x2x+1dx13101x+1dx
The RHS integral is straightforward but the LHS one poses more difficulties. The way forward with the former is to write the quadratic in the denominator as a sum of two squares:1310x+1(x12)2+34dx
This newly changed denominator can then be divided into the two parts of the numerator to yield:1310x(x12)2+34+13101(x12)2+34
I'm not going to go into the details here but, with the appropriate substitutions, the LHS of this integral can be integrated to a logarithm and the RHS to an arctan using the formulae:2tt2+1dt=ln|t2+1|+C and 1t2+1dt=tan1t+C
The indefinite integral for x/(x3+1) turns out to be: 16(ln|4x24x+4|+23tan1(23(x12)))13ln|x+1|+C
The definite integral for the limits of 0 and 1 is what was quoted earlier at the beginning of this post. In conclusion, I must say that Blogger is still inconsistently handling the display mode for LaTeX: sometimes it renders the LaTeX using the Javascript and other times it returns an image of the rendered code. Annoying.

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