Question Number 128956 by mathmax by abdo last updated on 11/Jan/21
$$\mathrm{calculate}\:\int_{\mathrm{0}} ^{\pi} \:\frac{\mathrm{sin}\left(\mathrm{2x}\right)}{\mathrm{3}+\mathrm{cos}\left(\mathrm{4x}\right)}\mathrm{dx} \\ $$
Answered by chengulapetrom last updated on 11/Jan/21
$${cos}\mathrm{4}{x}=\mathrm{2}{cos}^{\mathrm{2}} \mathrm{2}{x}−\mathrm{1} \\ $$$$\int_{\mathrm{0}} ^{\pi} \frac{{sin}\left(\mathrm{2}{x}\right)}{\mathrm{3}+{cos}\left(\mathrm{4}{x}\right)}{dx}=\int_{\mathrm{0}} ^{\pi} \frac{{sin}\left(\mathrm{2}{x}\right)}{\mathrm{2}+\mathrm{2}{cos}^{\mathrm{2}} \mathrm{2}{x}}{dx} \\ $$$$=−\frac{\mathrm{1}}{\mathrm{4}}\int_{\mathrm{0}} ^{\pi} \frac{−\mathrm{2}{sin}\left(\mathrm{2}{x}\right)}{\mathrm{1}+{cos}^{\mathrm{2}} \mathrm{2}{x}}{dx} \\ $$$${let}\:{t}={cos}\mathrm{2}{x}\Rightarrow{dt}=−\mathrm{2}{sin}\mathrm{2}{xdx} \\ $$$${x}=\mathrm{0}\Rightarrow{t}=\mathrm{1}\:{and}\:{x}=\pi\Rightarrow{t}=\mathrm{1} \\ $$$$\int_{\mathrm{1}} ^{\mathrm{1}} \frac{{dt}}{\mathrm{1}+{t}^{\mathrm{2}} }=\mathrm{0} \\ $$
Commented by chengulapetrom last updated on 11/Jan/21
$${am}\:{I}\:{correct}? \\ $$
Answered by Olaf last updated on 11/Jan/21
$$\Omega\:=\:\int_{\mathrm{0}} ^{\pi} \frac{\mathrm{sin}\left(\mathrm{2}{x}\right)}{\mathrm{3}+\mathrm{cos}\left(\mathrm{4}{x}\right)}{dx} \\ $$$$\Omega\:=\:\int_{\mathrm{0}} ^{\pi} \frac{\mathrm{sin}\left(\mathrm{2}{x}\right)}{\mathrm{3}+\mathrm{2cos}^{\mathrm{2}} \left(\mathrm{2}{x}\right)−\mathrm{1}}{dx} \\ $$$$\Omega\:=\:−\frac{\mathrm{1}}{\mathrm{4}}\int_{\mathrm{0}} ^{\pi} \frac{−\mathrm{2sin}\left(\mathrm{2}{x}\right)}{\mathrm{1}+\mathrm{cos}^{\mathrm{2}} \left(\mathrm{2}{x}\right)}{dx} \\ $$$$\Omega\:=\:−\frac{\mathrm{1}}{\mathrm{4}}\left[\mathrm{arctan}\left(\mathrm{cos}\left(\mathrm{2}{x}\right)\right)\right]_{\mathrm{0}} ^{\pi} \:=\:\mathrm{0} \\ $$
Answered by mathmax by abdo last updated on 11/Jan/21
$$\mathrm{let}\:\mathrm{I}=\int_{\mathrm{0}} ^{\pi} \:\frac{\mathrm{sin}\left(\mathrm{2x}\right)}{\mathrm{3}+\mathrm{cos}\left(\mathrm{4x}\right)}\mathrm{dx}\:\Rightarrow\mathrm{I}=_{\mathrm{2x}=\mathrm{t}} \:\:\int_{\mathrm{0}} ^{\mathrm{2}\pi} \:\frac{\mathrm{sint}}{\mathrm{3}+\mathrm{cos}\left(\mathrm{2t}\right)}\frac{\mathrm{dt}}{\mathrm{2}} \\ $$$$=\frac{\mathrm{1}}{\mathrm{2}}\int_{\mathrm{0}} ^{\mathrm{2}\pi} \:\frac{\mathrm{sint}}{\mathrm{3}+\mathrm{1}−\mathrm{2sin}^{\mathrm{2}} \mathrm{t}}\mathrm{dt}\:=\frac{\mathrm{1}}{\mathrm{2}}\int_{\mathrm{0}} ^{\mathrm{2}\pi} \:\frac{\mathrm{sint}}{\mathrm{4}−\mathrm{2sin}^{\mathrm{2}} \mathrm{t}}\mathrm{dt}\:=\frac{\mathrm{1}}{\mathrm{4}}\int_{\mathrm{0}} ^{\mathrm{2}\pi} \:\frac{\mathrm{sint}}{\mathrm{2}−\mathrm{sin}^{\mathrm{2}} \mathrm{t}}\mathrm{dt} \\ $$$$\Rightarrow\mathrm{4I}\:=\int_{\mathrm{0}} ^{\pi} \:\frac{\mathrm{sint}}{\mathrm{2}−\mathrm{sin}^{\mathrm{2}} \mathrm{t}}\mathrm{dt}\:+\int_{\pi} ^{\mathrm{2}\pi} \:\frac{\mathrm{sint}}{\mathrm{2}−\mathrm{sin}^{\mathrm{2}} \mathrm{t}}\mathrm{dt}\left(\rightarrow\mathrm{t}=\mathrm{x}+\pi\right. \\ $$$$=\int_{\mathrm{0}} ^{\pi} \:\frac{\mathrm{sint}}{\mathrm{2}−\mathrm{sin}^{\mathrm{2}} \mathrm{t}}\:−\int_{\mathrm{0}} ^{\pi} \:\frac{\mathrm{sinx}}{\mathrm{2}−\mathrm{sin}^{\mathrm{2}} \mathrm{x}}\mathrm{dx}\:=\mathrm{0}\:\Rightarrow\mathrm{4I}=\mathrm{0}\:\Rightarrow\mathrm{I}=\mathrm{0} \\ $$