Thursday, 27 August 2015

integration - Integral over the real axis using residues

I have to evaluate the following integral
xsinxx24x+8dx
I know that for such integrals we can use residues and
xsinxx24x+8dx=2πink=0res[f(z),ak]+lim
I see that f(z) = \frac{x\sin{x}}{x^2-4x+8} has one singularity in the upper half of the plane which is x = 2+2i and I have computed that res[f(z),2+2i] = \left(\frac{1}{2}-\frac{i}{2}\right)\sin{(2+2i)}, but I have problems evaluating the line integral. Usually we have shown that this goes to 0 as R gets large, but in this case it doesn't seem to be so. How can I go about evaluating the line integral? Thanks for any advice.

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