UNKNOWN Questions

Question Number 55775 by gunawan last updated on 04/Mar/19

$$\:\underset{\:\mathrm{0}} {\overset{\mathrm{1}} {\int}}\:\:\mid\mathrm{sin}\:\mathrm{2}\pi\:{x}\mid\:{dx}\:= \\$$

Commented bymaxmathsup by imad last updated on 04/Mar/19

$${let}\:{I}\:=\int_{\mathrm{0}} ^{\mathrm{1}} \mid{sin}\left(\mathrm{2}\pi{x}\right)\mid{dx}\:\:\Rightarrow\:{I}\:=_{\mathrm{2}\pi{x}\:={t}} \:\:\int_{\mathrm{0}} ^{\mathrm{2}\pi} \:\mid{sin}\left({t}\right)\mid\frac{{dt}}{\mathrm{2}\pi} \\$$ $$\left.=\frac{\mathrm{1}}{\mathrm{2}\pi}\left\{\:\int_{\mathrm{0}} ^{\pi} \:{sint}\:{dt}\:\right)\:+\int_{\pi} ^{\mathrm{2}\pi} \:\mid{sint}\mid{dt}\right\}\:\:{but}\:\int_{\pi} ^{\mathrm{2}\pi} \mid{sint}\mid{dt}\:=_{{t}\:=\pi+{u}} \int_{\mathrm{0}} ^{\pi} {sinu}\:{du}\:\Rightarrow \\$$ $${I}\:=\frac{\mathrm{1}}{\pi}\:\int_{\mathrm{0}} ^{\pi} \:{sint}\:{dt}\:=\frac{\mathrm{1}}{\pi}\left[−{cost}\right]_{\mathrm{0}} ^{\pi} \:=\frac{\mathrm{2}}{\pi}\:\Rightarrow\:{I}\:=\frac{\mathrm{2}}{\pi}\:. \\$$

Answered by tanmay.chaudhury50@gmail.com last updated on 04/Mar/19

$$\underset{\mathrm{0}} {\overset{\frac{\mathrm{1}}{\mathrm{2}}} {\int}}\mid{sin}\mathrm{2}\pi{x}\mid{dx}+\overset{\mathrm{1}} {\int}_{\frac{\mathrm{1}}{\mathrm{2}}} \mid{sin}\mathrm{2}\pi{x}\mid{dx} \\$$ $$\underset{\mathrm{0}} {\overset{\frac{\mathrm{1}}{\mathrm{2}}} {\int}}{sin}\mathrm{2}\pi{x}\:{dx}+\underset{\frac{\mathrm{1}}{\mathrm{2}}} {\overset{\mathrm{1}} {\int}}−{sin}\mathrm{2}\pi{x}\:{dx} \\$$ $$−\mid\frac{{cos}\mathrm{2}\pi{x}}{\mathrm{2}\pi}\mid_{\mathrm{0}} ^{\frac{\mathrm{1}}{\mathrm{2}}} +\mid\frac{{cos}\mathrm{2}\pi{x}}{\mathrm{2}\pi}\mid_{\frac{\mathrm{1}}{\mathrm{2}}} ^{\mathrm{1}} \: \\$$ $$=\frac{−{cos}\pi+{cos}\mathrm{0}}{\mathrm{2}\pi}+\frac{{cos}\mathrm{2}\pi−{cos}\pi}{\mathrm{2}\pi} \\$$ $$=\frac{\mathrm{1}+\mathrm{1}+\mathrm{1}+\mathrm{1}}{\mathrm{2}\pi}=\frac{\mathrm{2}}{\pi} \\$$ $${pls}\:{check}... \\$$