Thursday, May 30, 2019

fourier transform - Solution to this integral?



could anyone solve this integral ?



0exsin(x)cos(ax)x dx




well i have tried opening up the sin*cos using trigonometric identities but that didn't help so much


Answer



By the sine addition formulas, it is enough to compute
f(m)=+0sin(mx)xexdx
where f(0)=0 and by the dominated convergence theorem
f(m)=+0cos(mx)exdxIBP=11+m2
implying:
f(m)=+0sin(mx)xexdx=arctan(m)
and

+0sin(x)cos(ax)xexdx=arctan(1a)+arctan(1+a)2=12arctan2a2.


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