Question Number 77591 by naka3546 last updated on 08/Jan/20
$${x}\:+\:\frac{\mathrm{1}}{{x}}\:\:=\:\:\mathrm{1} \\ $$$${x}^{\mathrm{100}} \:+\:\frac{\mathrm{1}}{{x}^{\mathrm{100}} }\:\:=\:\:? \\ $$$${x}^{\mathrm{2020}} \:+\:\frac{\mathrm{1}}{{x}^{\mathrm{2020}} }\:\:=\:\:? \\ $$
Commented by jagoll last updated on 08/Jan/20
$$\frac{\mathrm{1}}{{x}^{\mathrm{2}} }+{x}^{\mathrm{2}} =−\mathrm{1} \\ $$$$\frac{\mathrm{1}}{{x}^{\mathrm{4}} }+{x}^{\mathrm{4}} =−\mathrm{1} \\ $$$$\frac{\mathrm{1}}{{x}^{\mathrm{8}} }+{x}^{\mathrm{8}} \:=\:−\mathrm{1} \\ $$$$\frac{\mathrm{1}}{{x}^{\mathrm{2}{n}} }+{x}^{\mathrm{2}{n}} =−\mathrm{1}\:{so}\:\frac{\mathrm{1}}{{x}^{\mathrm{100}} }+{x}^{\mathrm{100}} =−\mathrm{1} \\ $$
Commented by abdomathmax last updated on 08/Jan/20
$${x}+\frac{\mathrm{1}}{{x}}=\mathrm{1}\:\Rightarrow{x}^{\mathrm{2}} +\mathrm{1}={x}\:\Rightarrow{x}^{\mathrm{2}} −{x}+\mathrm{1}=\mathrm{0} \\ $$$$\Delta=\mathrm{1}−\mathrm{4}=−\mathrm{3}\:\Rightarrow{x}_{\mathrm{1}} =\frac{\mathrm{1}+{i}\sqrt{\mathrm{3}}}{\mathrm{2}}\:={e}^{\frac{{i}\pi}{\mathrm{3}}} \\ $$$${x}_{\mathrm{2}} =\frac{\mathrm{1}−{i}\sqrt{\mathrm{3}}}{\mathrm{2}}\:={e}^{−\frac{{i}\pi}{\mathrm{3}}} \: \\ $$$${x}={x}_{\mathrm{1}} \:\Rightarrow{x}^{\mathrm{100}} \:+{x}^{−\mathrm{100}} =\:{e}^{\frac{{i}\mathrm{100}\pi}{\mathrm{3}}} \:+{e}^{−\frac{{i}\mathrm{100}\pi}{\mathrm{3}}} \\ $$$$=\mathrm{2}{cos}\left(\frac{\mathrm{100}\pi}{\mathrm{3}}\right)\:=\mathrm{2}{cos}\left(\frac{\mathrm{99}\pi+\pi}{\mathrm{3}}\right)=\mathrm{2}{cos}\left(\mathrm{33}\pi\:+\frac{\pi}{\mathrm{3}}\right) \\ $$$$=\:\mathrm{2}{cos}\left(\pi+\frac{\pi}{\mathrm{3}}\right)=−\mathrm{2}×\frac{\mathrm{1}}{\mathrm{2}}\:=−\mathrm{1} \\ $$$${x}={x}_{\mathrm{2}} \:\Rightarrow{x}^{\mathrm{100}} \:+{x}^{−\mathrm{100}} =\:{e}^{−\frac{{i}\mathrm{100}\pi}{\mathrm{3}}} +{e}^{\frac{{i}\mathrm{100}\pi}{\mathrm{3}}} =−\mathrm{1} \\ $$$${for}\:{x}^{\mathrm{2020}} \:+\frac{\mathrm{1}}{{x}^{\mathrm{2020}} } \\ $$$${x}={x}_{\mathrm{1}} \:\Rightarrow{x}^{\mathrm{2020}} \:+{x}^{−\mathrm{2020}} ={e}^{{i}\frac{\mathrm{2020}\pi}{\mathrm{3}}} \:+{e}^{−\frac{{i}\mathrm{2020}\pi}{\mathrm{3}}} \\ $$$$=\mathrm{2}{cos}\left(\frac{\mathrm{2020}\pi}{\mathrm{3}}\right)=\mathrm{2}{cos}\left(\frac{\mathrm{2019}\pi+\pi}{\mathrm{3}}\right) \\ $$$$=\mathrm{2}{cos}\left(\frac{\mathrm{2019}\pi}{\mathrm{3}}+\frac{\pi}{\mathrm{3}}\right)=… \\ $$
Commented by naka3546 last updated on 08/Jan/20
$${n}=\:\mathrm{6}\:\:\Rightarrow\:\:{the}\:\:{answer}\:\:{is}\:\:{not}\:\:−\mathrm{1} \\ $$
Commented by jagoll last updated on 08/Jan/20
$${how}\:{you}\:{get}\:\mathrm{6}\:{sir}? \\ $$
Commented by naka3546 last updated on 08/Jan/20
$${prove}\:\:{it}\:\:{if}\:\:{the}\:\:{formula}\:\:{is}\:\:{right}\:\:{for}\:\:{all}\:\:{n}\:. \\ $$
Answered by MJS last updated on 08/Jan/20