Question Number 58195 by salaw2000 last updated on 19/Apr/19
$$\frac{\mathrm{1}}{\mathrm{x}}+\frac{\mathrm{1}}{\mathrm{y}}=\frac{\mathrm{3}}{\mathrm{4}} \\ $$$$\frac{\mathrm{x}^{\mathrm{2}} }{\mathrm{y}}+\frac{\mathrm{y}^{\mathrm{2}} }{\mathrm{x}}=\mathrm{9} \\ $$$$\mathrm{find}\:\mathrm{the}\:\mathrm{value}\:\mathrm{of}\:\mathrm{x}\:\mathrm{and}\:\mathrm{y} \\ $$
Answered by Kunal12588 last updated on 19/Apr/19
$$\frac{\mathrm{1}}{{x}}+\frac{\mathrm{1}}{{y}}=\frac{\mathrm{3}}{\mathrm{4}} \\ $$$$\Rightarrow\frac{{x}+{y}}{{xy}}=\frac{\mathrm{3}}{\mathrm{4}}\:\:\:\:\:\:\:\:\:\left(\mathrm{1}\right) \\ $$$$\Rightarrow{x}+{y}=\frac{\mathrm{3}{xy}}{\mathrm{4}}\:\:\:\:\:\:\left(\mathrm{2}\right) \\ $$$$\frac{{x}^{\mathrm{2}} }{{y}}+\frac{{y}^{\mathrm{2}} }{{x}}=\mathrm{9} \\ $$$$\Rightarrow\frac{{x}+{y}}{{xy}}\left({x}^{\mathrm{2}} −{xy}+{y}^{\mathrm{2}} \right)=\mathrm{9} \\ $$$${from}\:\left(\mathrm{1}\right) \\ $$$$\Rightarrow{x}^{\mathrm{2}} −{xy}+{y}^{\mathrm{2}} =\mathrm{12} \\ $$$$\Rightarrow\left({x}+{y}\right)^{\mathrm{2}} −\mathrm{3}{xy}=\mathrm{12}\:\:\:\:\:\:\:\:\left(\mathrm{3}\right) \\ $$$$\Rightarrow\left({x}−{y}\right)^{\mathrm{2}} +{xy}=\mathrm{12}\:\:\:\:\:\:\:\:\:\:\:\left(\mathrm{4}\right) \\ $$$${from}\:\left(\mathrm{3}\right) \\ $$$${x}+{y}=\pm\sqrt{\mathrm{3}{xy}+\mathrm{12}}\:\:\:\:\:\:\:\:\:\:\:\:\: \\ $$$${but}\:{from}\:\left(\mathrm{2}\right) \\ $$$$\frac{\mathrm{3}{xy}}{\mathrm{4}}=\sqrt{\mathrm{3}{xy}+\mathrm{12}} \\ $$$$\Rightarrow\mathrm{3}\centerdot\mathrm{3}{x}^{\mathrm{2}} {y}^{\mathrm{2}} =\mathrm{16}\left[\mathrm{3}\left({xy}+\mathrm{4}\right)\right] \\ $$$$\Rightarrow\mathrm{3}{x}^{\mathrm{2}} {y}^{\mathrm{2}} −\mathrm{16}{xy}−\mathrm{64}=\mathrm{0} \\ $$$$\Rightarrow{xy}=\frac{\mathrm{16}\pm\sqrt{\mathrm{16}^{\mathrm{2}} −\mathrm{4}\centerdot\mathrm{3}\centerdot\left(−\mathrm{64}\right)}}{\mathrm{2}\centerdot\mathrm{3}} \\ $$$$\Rightarrow{xy}=\frac{\mathrm{48}}{\mathrm{6}},\frac{−\mathrm{16}}{\mathrm{6}} \\ $$$$\Rightarrow{xy}=\mathrm{8},\frac{−\mathrm{8}}{\mathrm{3}} \\ $$$$\left({i}\right)\:{xy}=\mathrm{8} \\ $$$${putting}\:{xy}\:{in}\:\left(\mathrm{2}\right) \\ $$$${x}+{y}=\mathrm{6}\:\:\:\:\left(\mathrm{5}\right) \\ $$$${putting}\:{xy}\:{in}\:\left(\mathrm{4}\right) \\ $$$$\left({x}−{y}\right)^{\mathrm{2}} =\mathrm{12}−\mathrm{8} \\ $$$$\Rightarrow{x}−{y}=\pm\sqrt{\mathrm{4}} \\ $$$$\Rightarrow{x}−{y}=\mathrm{2}\:{or}\:{x}−{y}=−\mathrm{2} \\ $$$$\left({a}\right)\:\:{taking}\:{x}−{y}=\mathrm{2} \\ $$$${and}\:{comparing}\:{with}\:\left(\mathrm{5}\right) \\ $$$$\mathrm{1}^{{st}} \:{answer}\:\:\:{x}=\mathrm{4},{y}=\mathrm{2} \\ $$$$\left({b}\right)\:\:{taking}\:{x}−{y}=−\mathrm{2} \\ $$$${and}\:{comparing}\:{with}\:\left(\mathrm{5}\right) \\ $$$$\mathrm{2}^{{nd}} \:{answer}\:{x}=\mathrm{2},{y}=\mathrm{4} \\ $$$$\left({ii}\right)\:{xy}=\frac{−\mathrm{8}}{\mathrm{3}} \\ $$$${putting}\:{xy}\:{in}\:\left(\mathrm{2}\right) \\ $$$${x}+{y}=−\mathrm{2}\:\:\:\:\:\left(\mathrm{6}\right) \\ $$$${putting}\:{xy}\:{in}\:\left(\mathrm{4}\right) \\ $$$$\left({x}−{y}\right)^{\mathrm{2}} +{xy}=\mathrm{12} \\ $$$$\mathrm{3}\left({x}−{y}\right)^{\mathrm{2}} −\mathrm{8}=\mathrm{36} \\ $$$${x}−{y}=\pm\sqrt{\frac{\mathrm{44}}{\mathrm{3}}} \\ $$$$\left({a}\right)\:\:{taking}\:{x}−{y}=\sqrt{\frac{\mathrm{44}}{\mathrm{3}}} \\ $$$${and}\:{comparing}\:{with}\:\left(\mathrm{6}\right) \\ $$$$\mathrm{3}^{{rd}} \:{answer}\:{x}=−\mathrm{1}+\sqrt{\frac{\mathrm{11}}{\mathrm{3}}},{y}=−\mathrm{1}−\sqrt{\frac{\mathrm{11}}{\mathrm{3}}} \\ $$$$\left({b}\right)\:\:{taking}\:{x}−{y}=−\sqrt{\frac{\mathrm{44}}{\mathrm{3}}} \\ $$$${and}\:{comparing}\:{with}\:\left(\mathrm{6}\right) \\ $$$$\mathrm{4}^{{th}} \:{answer}\:{x}=−\mathrm{1}−\sqrt{\frac{\mathrm{11}}{\mathrm{3}}},{y}=−\mathrm{1}+\sqrt{\frac{\mathrm{11}}{\mathrm{3}}} \\ $$
Commented by salaw2000 last updated on 19/Apr/19
$$\mathrm{thank}\:\mathrm{you}\:\mathrm{very}\:\mathrm{much}\: \\ $$
Answered by MJS last updated on 19/Apr/19
$$\left(\mathrm{1}\right)\:{y}=\frac{\mathrm{4}{x}}{\mathrm{3}{x}−\mathrm{4}} \\ $$$$\left(\mathrm{2}\right)\:{x}^{\mathrm{3}} +{y}^{\mathrm{3}} −\mathrm{9}{xy}=\mathrm{0} \\ $$$$\left(\mathrm{1}\right)\:\mathrm{in}\:\left(\mathrm{2}\right)\:\Rightarrow \\ $$$${x}^{\mathrm{4}} −\mathrm{4}{x}^{\mathrm{3}} −\frac{\mathrm{20}}{\mathrm{3}}{x}^{\mathrm{2}} +\mathrm{32}{x}−\frac{\mathrm{64}}{\mathrm{3}}=\mathrm{0} \\ $$$$\mathrm{trying}\:\mathrm{factors}\:\mathrm{of}\:\pm\frac{\mathrm{64}}{\mathrm{3}}\:\mathrm{we}\:\mathrm{find} \\ $$$${x}_{\mathrm{1}} =\mathrm{2};\:{x}_{\mathrm{2}} =\mathrm{4} \\ $$$$\Rightarrow \\ $$$$\left({x}−\mathrm{2}\right)\left({x}−\mathrm{4}\right)\left({x}^{\mathrm{2}} +\mathrm{2}{x}−\frac{\mathrm{8}}{\mathrm{3}}\right)=\mathrm{0} \\ $$$$\Rightarrow\:{x}_{\mathrm{3}} =−\mathrm{1}−\frac{\sqrt{\mathrm{33}}}{\mathrm{3}};\:{x}_{\mathrm{4}} =−\mathrm{1}+\frac{\sqrt{\mathrm{33}}}{\mathrm{3}} \\ $$$$\Rightarrow\:\begin{pmatrix}{{x}}\\{{y}}\end{pmatrix}\in\left\{\begin{pmatrix}{\mathrm{2}}\\{\mathrm{4}}\end{pmatrix},\:\begin{pmatrix}{\mathrm{4}}\\{\mathrm{2}}\end{pmatrix},\:\begin{pmatrix}{−\mathrm{1}−\frac{\sqrt{\mathrm{33}}}{\mathrm{3}}}\\{−\mathrm{1}+\frac{\sqrt{\mathrm{33}}}{\mathrm{3}}}\end{pmatrix}\:,\:\begin{pmatrix}{−\mathrm{1}−\frac{\sqrt{\mathrm{33}}}{\mathrm{3}}}\\{−\mathrm{1}+\frac{\sqrt{\mathrm{33}}}{\mathrm{3}}}\end{pmatrix}\:\right\} \\ $$
Commented by Kunal12588 last updated on 19/Apr/19
Sir you are making me think why I did that
Commented by MJS last updated on 19/Apr/19
$$\mathrm{different}\:\mathrm{paths}\:\mathrm{lead}\:\mathrm{to}\:\mathrm{the}\:\mathrm{same}\:\mathrm{solutions}\:− \\ $$$$\mathrm{that}'\mathrm{s}\:\mathrm{a}\:\mathrm{good}\:\mathrm{thing}! \\ $$
Commented by salaw2000 last updated on 19/Apr/19
$$\mathrm{a}\:\mathrm{very}\:\mathrm{big}\:\mathrm{thanks}\:\mathrm{to}\:\mathrm{you} \\ $$$$ \\ $$$$ \\ $$$$ \\ $$