Menu Close

2-3-x-2-2-3-x-2-4-




Question Number 103914 by bobhans last updated on 18/Jul/20
(2+(√3))^x^2   + (2−(√3))^x^2   = 4
$$\left(\mathrm{2}+\sqrt{\mathrm{3}}\right)^{{x}^{\mathrm{2}} } \:+\:\left(\mathrm{2}−\sqrt{\mathrm{3}}\right)^{{x}^{\mathrm{2}} } \:=\:\mathrm{4}\: \\ $$
Commented by som(math1967) last updated on 18/Jul/20
let (2+(√3))^x^2  =a  ∴(2−(√3))^x^2  =(1/a)  ⇒a^2 −4a+1=0  ⇒a=(2±(√3))  (2+(√3))^x^2  =(2+(√3))  ⇒x=±1  (2+(√3))^x^2  =(2+(√3))^(−1)   x=i
$$\mathrm{let}\:\left(\mathrm{2}+\sqrt{\mathrm{3}}\right)^{\mathrm{x}^{\mathrm{2}} } =\mathrm{a} \\ $$$$\therefore\left(\mathrm{2}−\sqrt{\mathrm{3}}\right)^{\mathrm{x}^{\mathrm{2}} } =\frac{\mathrm{1}}{\mathrm{a}} \\ $$$$\Rightarrow\mathrm{a}^{\mathrm{2}} −\mathrm{4a}+\mathrm{1}=\mathrm{0} \\ $$$$\Rightarrow\mathrm{a}=\left(\mathrm{2}\pm\sqrt{\mathrm{3}}\right) \\ $$$$\left(\mathrm{2}+\sqrt{\mathrm{3}}\right)^{\mathrm{x}^{\mathrm{2}} } =\left(\mathrm{2}+\sqrt{\mathrm{3}}\right) \\ $$$$\Rightarrow\mathrm{x}=\pm\mathrm{1} \\ $$$$\left(\mathrm{2}+\sqrt{\mathrm{3}}\right)^{\mathrm{x}^{\mathrm{2}} } =\left(\mathrm{2}+\sqrt{\mathrm{3}}\right)^{−\mathrm{1}} \\ $$$$\mathrm{x}=\mathrm{i} \\ $$
Commented by bemath last updated on 18/Jul/20
thank you both
$${thank}\:{you}\:{both} \\ $$
Answered by OlafThorendsen last updated on 18/Jul/20
X = (2+(√3))^x^2    (2−(√3))^x^2   = ((1/(2+(√3))))^x^2  = (1/((2+(√3))^x^2  )) = (1/X)  then X+(1/X) = 4  X^2 −4X+1 = 0  Δ = (−4)^2 −4(1)(1) = 12  X = ((−(−4)±(√(12)))/(2(1))) = 2±(√3)  1) X = 2+(√3)  x^2  = 1, x=±1  2) X = 2−(√3)  2−(√3) = (2+(√3))^x^2    (1/(2+(√3))) = (2+(√3))^x^2    (2+(√3))^(x^2 +1)  = 1  x^2 +1 = 0  x = ±i  S = {±1;±i}
$${X}\:=\:\left(\mathrm{2}+\sqrt{\mathrm{3}}\right)^{{x}^{\mathrm{2}} } \\ $$$$\left(\mathrm{2}−\sqrt{\mathrm{3}}\right)^{{x}^{\mathrm{2}} } \:=\:\left(\frac{\mathrm{1}}{\mathrm{2}+\sqrt{\mathrm{3}}}\right)^{{x}^{\mathrm{2}} } =\:\frac{\mathrm{1}}{\left(\mathrm{2}+\sqrt{\mathrm{3}}\right)^{{x}^{\mathrm{2}} } }\:=\:\frac{\mathrm{1}}{{X}} \\ $$$$\mathrm{then}\:{X}+\frac{\mathrm{1}}{{X}}\:=\:\mathrm{4} \\ $$$${X}^{\mathrm{2}} −\mathrm{4}{X}+\mathrm{1}\:=\:\mathrm{0} \\ $$$$\Delta\:=\:\left(−\mathrm{4}\right)^{\mathrm{2}} −\mathrm{4}\left(\mathrm{1}\right)\left(\mathrm{1}\right)\:=\:\mathrm{12} \\ $$$${X}\:=\:\frac{−\left(−\mathrm{4}\right)\pm\sqrt{\mathrm{12}}}{\mathrm{2}\left(\mathrm{1}\right)}\:=\:\mathrm{2}\pm\sqrt{\mathrm{3}} \\ $$$$\left.\mathrm{1}\right)\:{X}\:=\:\mathrm{2}+\sqrt{\mathrm{3}} \\ $$$${x}^{\mathrm{2}} \:=\:\mathrm{1},\:{x}=\pm\mathrm{1} \\ $$$$\left.\mathrm{2}\right)\:{X}\:=\:\mathrm{2}−\sqrt{\mathrm{3}} \\ $$$$\mathrm{2}−\sqrt{\mathrm{3}}\:=\:\left(\mathrm{2}+\sqrt{\mathrm{3}}\right)^{{x}^{\mathrm{2}} } \\ $$$$\frac{\mathrm{1}}{\mathrm{2}+\sqrt{\mathrm{3}}}\:=\:\left(\mathrm{2}+\sqrt{\mathrm{3}}\right)^{{x}^{\mathrm{2}} } \\ $$$$\left(\mathrm{2}+\sqrt{\mathrm{3}}\right)^{{x}^{\mathrm{2}} +\mathrm{1}} \:=\:\mathrm{1} \\ $$$${x}^{\mathrm{2}} +\mathrm{1}\:=\:\mathrm{0} \\ $$$${x}\:=\:\pm{i} \\ $$$$\mathscr{S}\:=\:\left\{\pm\mathrm{1};\pm{i}\right\} \\ $$
Answered by Rauny last updated on 18/Jul/20
(2+(√3))^x^2   + (2−(√3))^x^2   = 4   t:=2+(√3)  (1/(2+(√3)))=((2−(√3))/((2+(√3))(2−(√3))))=2−(√3)  ∴2−(√3)=(1/t)  t^x^2  +t^(−x^2 ) −4=0  u:=t^x^2   (∀u>0)  u−4+(1/u)=0  u^2 −4u+1=0  u=2±(√3)  t^x^2  =2±(√3)  x^2 =log_(2+(√3))  (2±(√3))  if u=2+(√3),  x^2 =1  x=±1  if u=2−(√3),  x^2 =log_(2+(√3))  (1/(2+(√3)))=−1 (∵2−(√3)=(1/(2+(√3))))  x=±i  ∴x=±1 or ±i ■
$$\left(\mathrm{2}+\sqrt{\mathrm{3}}\right)^{{x}^{\mathrm{2}} } \:+\:\left(\mathrm{2}−\sqrt{\mathrm{3}}\right)^{{x}^{\mathrm{2}} } \:=\:\mathrm{4}\: \\ $$$${t}:=\mathrm{2}+\sqrt{\mathrm{3}} \\ $$$$\frac{\mathrm{1}}{\mathrm{2}+\sqrt{\mathrm{3}}}=\frac{\mathrm{2}−\sqrt{\mathrm{3}}}{\left(\mathrm{2}+\sqrt{\mathrm{3}}\right)\left(\mathrm{2}−\sqrt{\mathrm{3}}\right)}=\mathrm{2}−\sqrt{\mathrm{3}} \\ $$$$\therefore\mathrm{2}−\sqrt{\mathrm{3}}=\frac{\mathrm{1}}{{t}} \\ $$$${t}^{{x}^{\mathrm{2}} } +{t}^{−{x}^{\mathrm{2}} } −\mathrm{4}=\mathrm{0} \\ $$$${u}:={t}^{{x}^{\mathrm{2}} } \:\left(\forall{u}>\mathrm{0}\right) \\ $$$${u}−\mathrm{4}+\frac{\mathrm{1}}{{u}}=\mathrm{0} \\ $$$${u}^{\mathrm{2}} −\mathrm{4}{u}+\mathrm{1}=\mathrm{0} \\ $$$${u}=\mathrm{2}\pm\sqrt{\mathrm{3}} \\ $$$${t}^{{x}^{\mathrm{2}} } =\mathrm{2}\pm\sqrt{\mathrm{3}} \\ $$$${x}^{\mathrm{2}} =\mathrm{log}_{\mathrm{2}+\sqrt{\mathrm{3}}} \:\left(\mathrm{2}\pm\sqrt{\mathrm{3}}\right) \\ $$$$\mathrm{if}\:{u}=\mathrm{2}+\sqrt{\mathrm{3}}, \\ $$$${x}^{\mathrm{2}} =\mathrm{1} \\ $$$${x}=\pm\mathrm{1} \\ $$$$\mathrm{if}\:{u}=\mathrm{2}−\sqrt{\mathrm{3}}, \\ $$$${x}^{\mathrm{2}} =\mathrm{log}_{\mathrm{2}+\sqrt{\mathrm{3}}} \:\frac{\mathrm{1}}{\mathrm{2}+\sqrt{\mathrm{3}}}=−\mathrm{1}\:\left(\because\mathrm{2}−\sqrt{\mathrm{3}}=\frac{\mathrm{1}}{\mathrm{2}+\sqrt{\mathrm{3}}}\right) \\ $$$${x}=\pm{i} \\ $$$$\therefore{x}=\pm\mathrm{1}\:\mathrm{or}\:\pm{i}\:\blacksquare \\ $$

Leave a Reply

Your email address will not be published. Required fields are marked *