Question Number 92993 by mathmax by abdo last updated on 10/May/20 | ||
$${calculate}\:\:\int_{−\infty} ^{+\infty} \:\frac{{cos}\left({cosx}−{sinx}\right)}{{x}^{\mathrm{2}} \:+\mathrm{4}}{dx} \\ $$ | ||
Commented by mathmax by abdo last updated on 10/May/20 | ||
$${I}\:=\int_{−\infty} ^{+\infty} \:\frac{{cos}\left({cosx}−{sinx}\right)}{{x}^{\mathrm{2}} +\mathrm{4}}{dx}\:\Rightarrow\:{I}\:=\int_{−\infty} ^{+\infty} \:\frac{{cos}\left(\sqrt{\mathrm{2}}{cos}\left({x}+\frac{\pi}{\mathrm{4}}\right)\right)}{{x}^{\mathrm{2}} \:+\mathrm{4}}{dx} \\ $$$$=_{{x}+\frac{\pi}{\mathrm{4}}={t}} \:\:\:\int_{−\infty} ^{+\infty} \:\frac{{cos}\left(\sqrt{\mathrm{2}}{t}\right)}{\left({t}−\frac{\pi}{\mathrm{4}}\right)^{\mathrm{2}} \:+\mathrm{4}}{dt}\:={Re}\left(\int_{−\infty} ^{+\infty} \:\frac{{e}^{{i}\sqrt{\mathrm{2}}{t}} }{\left({t}−\frac{\pi}{\mathrm{4}}\right)^{\mathrm{2}} +\mathrm{4}}\right) \\ $$$${let}\:\varphi\left({z}\right)\:=\frac{{e}^{{iz}\sqrt{\mathrm{2}}} }{\left({z}−\frac{\pi}{\mathrm{4}}\right)^{\mathrm{2}} +\mathrm{4}}\:\:\:{poles}\:{of}\:\varphi? \\ $$$$\varphi\left({z}\right)\:=\frac{{e}^{{iz}\sqrt{\mathrm{2}}} }{\left({z}−\frac{\pi}{\mathrm{4}}\right)^{\mathrm{2}} −\left(\mathrm{2}{i}\right)^{\mathrm{2}} }\:=\frac{{e}^{{iz}\sqrt{\mathrm{2}}} }{\left({z}−\frac{\pi}{\mathrm{4}}−\mathrm{2}{i}\right)\left({z}−\frac{\pi}{\mathrm{4}}+\mathrm{2}{i}\right)} \\ $$$${so}\:{the}\:{poles}\:{of}\:\varphi\:{are}\:\frac{\pi}{\mathrm{4}}+\mathrm{2}{i}\:{and}\:\frac{\pi}{\mathrm{4}}−\mathrm{2}{i} \\ $$$$\int_{−\infty} ^{+\infty} \:\varphi\left({z}\right){dz}\:=\mathrm{2}{i}\pi\:{Res}\left(\varphi,\frac{\pi}{\mathrm{4}}+\mathrm{2}{i}\right)\:=\mathrm{2}{i}\pi\frac{{e}^{{i}\sqrt{\mathrm{2}}\left(\frac{\pi}{\mathrm{4}}+\mathrm{2}{i}\right)} }{\mathrm{4}{i}} \\ $$$$=\frac{\pi}{\mathrm{2}}\:{e}^{\frac{{i}\pi\sqrt{\mathrm{2}}}{\mathrm{4}}} \:{e}^{−\mathrm{2}\sqrt{\mathrm{2}}} =\frac{\pi}{\mathrm{2}}{e}^{−\mathrm{2}\sqrt{\mathrm{2}}} \:\:\left\{{cos}\left(\frac{\pi\sqrt{\mathrm{2}}}{\mathrm{4}}\right)+{isin}\left(\frac{\pi\sqrt{\mathrm{2}}}{\mathrm{4}}\right)\right\}\:\Rightarrow \\ $$$${I}\:=\frac{\pi}{\mathrm{2}}{e}^{−\mathrm{2}\sqrt{\mathrm{2}}} \:{cos}\left(\frac{\pi\sqrt{\mathrm{2}}}{\mathrm{4}}\right) \\ $$$$ \\ $$ | ||