Question Number 78974 by ~blr237~ last updated on 22/Jan/20
$$\left\{_{\mathrm{3e}^{\mathrm{2x}} −\mathrm{y}^{\mathrm{2}} =\mathrm{2}\:\:\:} ^{\mathrm{e}^{\mathrm{x}} −\mathrm{y}^{\mathrm{2}} =\mathrm{2lny}−\mathrm{x}} \right. \\ $$$$\mathrm{solve}\:\mathrm{the}\:\mathrm{system}\:\mathrm{on}\:\mathbb{R}\ast\mathbb{R} \\ $$
Commented by jagoll last updated on 22/Jan/20
$$\mathrm{3}\left(\mathrm{e}^{\mathrm{x}} \right)^{\mathrm{2}} =\mathrm{2}+\mathrm{y}^{\mathrm{2}} \Rightarrow\mathrm{e}^{\mathrm{x}} =\:\sqrt{\frac{\mathrm{2}+\mathrm{y}^{\mathrm{2}} }{\mathrm{3}}} \\ $$$$\mathrm{x}=\:\mathrm{ln}\left(\sqrt{\frac{\mathrm{2}+\mathrm{y}^{\mathrm{2}} }{\mathrm{3}}}\right) \\ $$$$\sqrt{\frac{\mathrm{2}+\mathrm{y}^{\mathrm{2}} }{\mathrm{3}}}−\mathrm{y}^{\mathrm{2}} =\:\mathrm{lny}^{\mathrm{2}} −\mathrm{ln}\left(\sqrt{\frac{\mathrm{2}+\mathrm{y}^{\mathrm{2}} }{\mathrm{3}}}\right) \\ $$
Commented by jagoll last updated on 22/Jan/20
$$\mathrm{let}\:\sqrt{\frac{\mathrm{2}+\mathrm{y}^{\mathrm{2}} }{\mathrm{3}}}=\mathrm{t}\:\Rightarrow\mathrm{y}^{\mathrm{2}} \:=\:\mathrm{3t}^{\mathrm{2}} −\mathrm{2} \\ $$$$\mathrm{t}−\left(\mathrm{3t}^{\mathrm{2}} −\mathrm{2}\right)=\mathrm{ln}\left(\mathrm{3t}^{\mathrm{2}} −\mathrm{2}\right)−\mathrm{ln}\left(\mathrm{t}\right) \\ $$
Commented by MJS last updated on 22/Jan/20
give a few more examples where we can use this solving method (let me call it "finding a function") so we can practice and learn something.
to post well constructed problems to test if somebody will find the key can be fun but I usually refuse to solve riddles.
Answered by MJS last updated on 22/Jan/20
$${x}=\mathrm{0}\wedge{y}=\mathrm{1} \\ $$
Commented by jagoll last updated on 22/Jan/20
$$\mathrm{how}\:\mathrm{to}\:\mathrm{get}\:\mathrm{the}\:\mathrm{result}\:\mathrm{sir}? \\ $$
Commented by mr W last updated on 22/Jan/20
$${such}\:{stupid}\:{equations}\:{can}\:{not}\:{be} \\ $$$${solved}\:{analytically}.\:{you}\:{should}\:“{see}'' \\ $$$${the}\:{solution}\:{directly}.\:{when}\:{a}\:{value}\:{in} \\ $$$${the}\:{equation}\:{is}\:{changed},\:{the}\:{solution} \\ $$$${can}\:{not}\:{be}\:“{seen}''\:{anymore}.\:{since}\:{the} \\ $$$${solution}\:{is}\:{not}\:{solved},\:{but}\:“{seen}'', \\ $$$${you}\:{learn}\:{nothing}\:{through}\:{such}\: \\ $$$${questions}.\:{so}\:{just}\:{forget}\:{it},\:{if}\:{you} \\ $$$${don}'{t}\:“{see}''\:{the}\:{solution}\:{at}\:{first}\:{glance}. \\ $$
Commented by mr W last updated on 22/Jan/20
$${this}\:{is}\:{the}\:{same}\:{as}\:{questions}\:{in}\:{which} \\ $$$${four}\:{numbers}\:{are}\:{given}\:{and}\:{you}\:{should} \\ $$$${find}\:{the}\:{fifth}\:{number}.\:{questions}\:{of} \\ $$$${this}\:{kind}\:{are}\:{games},\:{not}\:{mathematics}. \\ $$
Answered by mind is power last updated on 22/Jan/20
$${y}>\mathrm{0} \\ $$$$\Rightarrow\mathrm{2}{x}={ln}\left(\frac{\mathrm{2}+{y}^{\mathrm{2}} }{\mathrm{3}}\right) \\ $$$${x}=\frac{\mathrm{1}}{\mathrm{2}}{ln}\left(\frac{\mathrm{2}+{y}^{\mathrm{2}} }{\mathrm{3}}\right) \\ $$$$\Rightarrow{first}\:{Equation}\Leftrightarrow\sqrt{\frac{\mathrm{2}+{y}^{\mathrm{2}} }{\mathrm{3}}}−{y}^{\mathrm{2}} =\mathrm{2}{ln}\left({y}\right)−\frac{{ln}\left(\frac{\mathrm{2}+{y}^{\mathrm{2}} }{\mathrm{3}}\right)}{\mathrm{2}} \\ $$$$\Leftrightarrow\mathrm{2}{ln}\left({y}\right)+{y}^{\mathrm{2}} =\sqrt{\frac{\mathrm{2}+{y}^{\mathrm{2}} }{\mathrm{3}}}+{ln}\left(\sqrt{\frac{\mathrm{2}+{y}^{\mathrm{2}} }{\mathrm{3}}}\right) \\ $$$$\Leftrightarrow{ln}\left({y}^{\mathrm{2}} \right)+{y}^{\mathrm{2}} =\sqrt{\frac{\mathrm{2}+{y}^{\mathrm{2}} }{\mathrm{3}}}+{ln}\left(\sqrt{\frac{\mathrm{2}+{y}^{\mathrm{2}} }{\mathrm{3}}}\right)…._{} {E} \\ $$$${let}\:{f}\left({x}\right)={ln}\left({x}\right)+{x},{x}>\mathrm{0} \\ $$$${E}\Leftrightarrow{f}\left({y}^{\mathrm{2}} \right)={f}\left(\sqrt{\frac{\mathrm{2}+{y}^{\mathrm{2}} }{\mathrm{3}}}\right) \\ $$$${f}'\left({x}\right)=\frac{\mathrm{1}}{{x}}+\mathrm{1}>\mathrm{1}\:\:{increase}\:{Function}\Rightarrow{Injective}\:{one} \\ $$$$\Rightarrow{f}\left({y}^{\mathrm{2}} \right)={f}\left(\sqrt{\frac{\mathrm{2}+{y}^{\mathrm{2}} }{\mathrm{3}}}\right)\Leftrightarrow{y}^{\mathrm{2}} =\sqrt{\frac{\mathrm{2}+{y}^{\mathrm{2}} }{\mathrm{3}}}\Leftrightarrow\mathrm{3}{y}^{\mathrm{4}} −{y}^{\mathrm{2}} −\mathrm{2}=\mathrm{0} \\ $$$${y}^{\mathrm{2}} ,{root}\:{of}\:\mathrm{3}{X}^{\mathrm{2}} −{X}−\mathrm{2}=\mathrm{0} \\ $$$${X}\in\left\{−\frac{\mathrm{2}}{\mathrm{3}},\mathrm{1}\right\},\Rightarrow{y}^{\mathrm{2}} =\mathrm{1}\Rightarrow{y}=\mathrm{1} \\ $$$${x}=\frac{\mathrm{1}}{\mathrm{2}}{ln}\left(\frac{\mathrm{2}+{y}^{\mathrm{2}} }{\mathrm{3}}\right)=\frac{{ln}\left(\frac{\mathrm{3}}{\mathrm{3}}\right)}{\mathrm{2}}=\mathrm{0} \\ $$$$\Rightarrow\left({x},{y}\right)\in\left\{\left(\mathrm{0},\mathrm{1}\right)\right\} \\ $$$$ \\ $$
Commented by ~blr237~ last updated on 22/Jan/20
$$\mathrm{Thank}\:\mathrm{sir}\:. \\ $$
Commented by mind is power last updated on 22/Jan/20
$${y}'{re}\:{Welcom} \\ $$
Answered by ~blr237~ last updated on 22/Jan/20
$$\mathrm{with}\:\mathrm{the}\:\mathrm{first}\:\mathrm{equation}\:\mathrm{you}\:\mathrm{reach}\:\mathrm{to}\:\left(\:\mathrm{on}\:\mathrm{the}\:\mathrm{strain}\:\mathrm{y}>\mathrm{0}\right) \\ $$$$\mathrm{e}^{\mathrm{x}} +\mathrm{x}=\mathrm{lny}^{\mathrm{2}} +\mathrm{e}^{\mathrm{lny}^{\mathrm{2}} } \:\:\Leftrightarrow\:\mathrm{f}\left(\mathrm{x}\right)=\mathrm{f}\left(\mathrm{lny}^{\mathrm{2}} \right)\:\:\mathrm{with}\:\mathrm{f}\left(\mathrm{t}\right)=\mathrm{e}^{\mathrm{t}} +\mathrm{t} \\ $$$$\mathrm{f}\:\mathrm{is}\:\mathrm{continue}\:\mathrm{and}\:\mathrm{strictly}\:\mathrm{increasing}\:\mathrm{on}\:\mathbb{R}\:\left(\:\mathrm{f}'\left(\mathrm{t}\right)=\mathrm{e}^{\mathrm{t}} +\mathrm{1}\:>\mathrm{0}\:\right) \\ $$$$\mathrm{so}\:\mathrm{f}\:\mathrm{is}\:\mathrm{injective}\:\left(\:\mathrm{cause}\:\mathrm{also}\:\mathrm{a}\:\mathrm{bijection}\:\mathrm{from}\:\mathbb{R}\rightarrow\mathbb{R}\:\:\right) \\ $$$$\mathrm{then}\:\:\mathrm{x}=\mathrm{lny}^{\mathrm{2}} \:\:\mathrm{or}\:\mathrm{y}^{\mathrm{2}} =\mathrm{e}^{\mathrm{x}} \\ $$$$\mathrm{when}\:\mathrm{replacing}\:\mathrm{on}\:\mathrm{the}\:\mathrm{second}\:\mathrm{we}\:\mathrm{have} \\ $$$$\mathrm{3}\left(\mathrm{e}^{\mathrm{x}} \right)^{\mathrm{2}} −\mathrm{e}^{\mathrm{x}} −\mathrm{2}=\mathrm{0}\: \\ $$$$\mathrm{then}\:\:\mathrm{e}^{\mathrm{x}} =\mathrm{1}\:\mathrm{or}\:\mathrm{e}^{\mathrm{x}} =−\frac{\mathrm{2}}{\mathrm{3}}\: \\ $$$$\mathrm{finally}\:\:\mathrm{x}=\mathrm{0}\:\mathrm{and}\:\mathrm{y}=\mathrm{1}\:\mathrm{cause}\:\mathrm{the}\:\mathrm{strain}\:\mathrm{was}\:\mathrm{y}>\mathrm{0} \\ $$$$ \\ $$