Question Number 210933 by mnjuly1970 last updated on 22/Aug/24
$$ \\ $$$$ \\ $$$$ \\ $$$$ \\ $$$$\:\:\:\:\:\:\mathrm{I}=\:\int_{\mathrm{0}} ^{\:\frac{\pi}{\mathrm{2}}\:} \frac{\:{sin}\left(\:\mathrm{25}{x}\:\right)}{{sinx}}\:{dx}=? \\ $$$$ \\ $$$$ \\ $$
Answered by Ghisom last updated on 22/Aug/24
$$\frac{\mathrm{sin}\:\mathrm{25}{x}}{\mathrm{sin}\:{x}}=\frac{\frac{\mathrm{e}^{\mathrm{25i}{x}} −\mathrm{e}^{−\mathrm{25i}{x}} }{\mathrm{2i}}}{\frac{\mathrm{e}^{\mathrm{i}{x}} −\mathrm{e}^{−\mathrm{i}{x}} }{\mathrm{2i}}}= \\ $$$$=\mathrm{1}+\underset{{n}=\mathrm{1}} {\overset{\mathrm{12}} {\sum}}\left(\mathrm{e}^{\mathrm{2}{nx}\mathrm{i}} +\mathrm{e}^{−\mathrm{2}{nx}\mathrm{i}} \right)\:= \\ $$$$=\mathrm{1}+\mathrm{2}\underset{{n}=\mathrm{1}} {\overset{\mathrm{12}} {\sum}}\:\mathrm{cos}\:\mathrm{2}{nx} \\ $$$$\Rightarrow \\ $$$$\underset{\mathrm{0}} {\overset{\pi/\mathrm{2}} {\int}}\:\frac{\mathrm{sin}\:\mathrm{25}{x}}{\mathrm{sin}\:{x}}{dx}=\left[{x}+\underset{{n}=\mathrm{1}} {\overset{\mathrm{12}} {\sum}}\:\frac{\mathrm{sin}\:\mathrm{2}{nx}}{{n}}\right]_{\mathrm{0}} ^{\pi/\mathrm{2}} = \\ $$$$=\frac{\pi}{\mathrm{2}} \\ $$
Commented by mnjuly1970 last updated on 23/Aug/24
$${thanks}\:{alot}\:{sir}\:.{excellent} \\ $$