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Question Number 74244 by Maclaurin Stickker last updated on 20/Nov/19

Find x  x^(log_(4 ) (√x)) =x^(log_4 x) −2

$${Find}\:{x} \\ $$$${x}^{{log}_{\mathrm{4}\:} \sqrt{{x}}} ={x}^{{log}_{\mathrm{4}} {x}} −\mathrm{2} \\ $$

Answered by MJS last updated on 20/Nov/19

t=x^(log_4  (√x)) =x^((ln (√x))/(ln 4)) =x^((ln x)/(4ln 2))   ln t =((ln x)/(4ln 2))ln x =(((ln x)^2 )/(4ln 2))  ⇒ x=e^(2(√(ln 2 ln t)))   x^(log_4  (√x)) =t  x^(log_4  x) =t^2   t=t^2 −2  t^2 −t−2=0  ⇒ t=−1∨t=2 but for x∈R we need t>1  ⇒ t=2 ⇒ x=4

$${t}={x}^{\mathrm{log}_{\mathrm{4}} \:\sqrt{{x}}} ={x}^{\frac{\mathrm{ln}\:\sqrt{{x}}}{\mathrm{ln}\:\mathrm{4}}} ={x}^{\frac{\mathrm{ln}\:{x}}{\mathrm{4ln}\:\mathrm{2}}} \\ $$$$\mathrm{ln}\:{t}\:=\frac{\mathrm{ln}\:{x}}{\mathrm{4ln}\:\mathrm{2}}\mathrm{ln}\:{x}\:=\frac{\left(\mathrm{ln}\:{x}\right)^{\mathrm{2}} }{\mathrm{4ln}\:\mathrm{2}} \\ $$$$\Rightarrow\:{x}=\mathrm{e}^{\mathrm{2}\sqrt{\mathrm{ln}\:\mathrm{2}\:\mathrm{ln}\:{t}}} \\ $$$${x}^{\mathrm{log}_{\mathrm{4}} \:\sqrt{{x}}} ={t} \\ $$$${x}^{\mathrm{log}_{\mathrm{4}} \:{x}} ={t}^{\mathrm{2}} \\ $$$${t}={t}^{\mathrm{2}} −\mathrm{2} \\ $$$${t}^{\mathrm{2}} −{t}−\mathrm{2}=\mathrm{0} \\ $$$$\Rightarrow\:{t}=−\mathrm{1}\vee{t}=\mathrm{2}\:\mathrm{but}\:\mathrm{for}\:{x}\in\mathbb{R}\:\mathrm{we}\:\mathrm{need}\:{t}>\mathrm{1} \\ $$$$\Rightarrow\:{t}=\mathrm{2}\:\Rightarrow\:{x}=\mathrm{4} \\ $$

Answered by mr W last updated on 20/Nov/19

x>0  let u=x^(log_4  (√x)) =x^((1/2)log_4  x) =(√x^(log_4  x) )>0  ⇒u=u^2 −2  ⇒u^2 −u−2=0  ⇒(u−2)(u+1)=0  ⇒u=2, −1  with u=2:  (√x^(log_4  x) )=2  x^(log_4  x) =4  x^((ln x)/(ln 4)) =4  ((ln x)/(ln 4))=((ln 4)/(ln x))  (ln x)^2 =(ln 4)^2   ln x=±ln 4=ln 4^(±1)   ⇒x=4^(±1) =4 or (1/4)

$${x}>\mathrm{0} \\ $$$${let}\:{u}={x}^{\mathrm{log}_{\mathrm{4}} \:\sqrt{{x}}} ={x}^{\frac{\mathrm{1}}{\mathrm{2}}\mathrm{log}_{\mathrm{4}} \:{x}} =\sqrt{{x}^{\mathrm{log}_{\mathrm{4}} \:{x}} }>\mathrm{0} \\ $$$$\Rightarrow{u}={u}^{\mathrm{2}} −\mathrm{2} \\ $$$$\Rightarrow{u}^{\mathrm{2}} −{u}−\mathrm{2}=\mathrm{0} \\ $$$$\Rightarrow\left({u}−\mathrm{2}\right)\left({u}+\mathrm{1}\right)=\mathrm{0} \\ $$$$\Rightarrow{u}=\mathrm{2},\:−\mathrm{1} \\ $$$${with}\:{u}=\mathrm{2}: \\ $$$$\sqrt{{x}^{\mathrm{log}_{\mathrm{4}} \:{x}} }=\mathrm{2} \\ $$$${x}^{\mathrm{log}_{\mathrm{4}} \:{x}} =\mathrm{4} \\ $$$${x}^{\frac{\mathrm{ln}\:{x}}{\mathrm{ln}\:\mathrm{4}}} =\mathrm{4} \\ $$$$\frac{\mathrm{ln}\:{x}}{\mathrm{ln}\:\mathrm{4}}=\frac{\mathrm{ln}\:\mathrm{4}}{\mathrm{ln}\:{x}} \\ $$$$\left(\mathrm{ln}\:{x}\right)^{\mathrm{2}} =\left(\mathrm{ln}\:\mathrm{4}\right)^{\mathrm{2}} \\ $$$$\mathrm{ln}\:{x}=\pm\mathrm{ln}\:\mathrm{4}=\mathrm{ln}\:\mathrm{4}^{\pm\mathrm{1}} \\ $$$$\Rightarrow{x}=\mathrm{4}^{\pm\mathrm{1}} =\mathrm{4}\:{or}\:\frac{\mathrm{1}}{\mathrm{4}} \\ $$

Commented by MJS last updated on 20/Nov/19

same path we share...  but the given equation also has the solution  x=(1/4)  how to get it?

$$\mathrm{same}\:\mathrm{path}\:\mathrm{we}\:\mathrm{share}... \\ $$$$\mathrm{but}\:\mathrm{the}\:\mathrm{given}\:\mathrm{equation}\:\mathrm{also}\:\mathrm{has}\:\mathrm{the}\:\mathrm{solution} \\ $$$${x}=\frac{\mathrm{1}}{\mathrm{4}} \\ $$$$\mathrm{how}\:\mathrm{to}\:\mathrm{get}\:\mathrm{it}? \\ $$

Commented by mr W last updated on 20/Nov/19

two solutions indeed. see above.

$${two}\:{solutions}\:{indeed}.\:{see}\:{above}. \\ $$

Commented by Maclaurin Stickker last updated on 21/Nov/19

Sir, I added 2 by the sides and applied  log_2  in both sides. Is this correct?  I got the same results

$${Sir},\:{I}\:{added}\:\mathrm{2}\:{by}\:{the}\:{sides}\:{and}\:{applied} \\ $$$${log}_{\mathrm{2}} \:{in}\:{both}\:{sides}.\:{Is}\:{this}\:{correct}? \\ $$$${I}\:{got}\:{the}\:{same}\:{results} \\ $$

Commented by MJS last updated on 21/Nov/19

doesn′t sound correct to me, please post it

$$\mathrm{doesn}'\mathrm{t}\:\mathrm{sound}\:\mathrm{correct}\:\mathrm{to}\:\mathrm{me},\:\mathrm{please}\:\mathrm{post}\:\mathrm{it} \\ $$

Commented by Maclaurin Stickker last updated on 21/Nov/19

x^(log_4 (√x)) +2=x^(log_4 x)   log_2 (x^(log_4 (√x)) )+log_2 2=log_2 (x^(log_4 x) )  ....  (1/4)log_2 x^2 =(1/2)log_2 x^2 −1  ....  log_2 x^2 =4  log_2 x=2  and  log_2 x=−2  x=4  or x=(1/4)

$${x}^{{log}_{\mathrm{4}} \sqrt{{x}}} +\mathrm{2}={x}^{{log}_{\mathrm{4}} {x}} \\ $$$${log}_{\mathrm{2}} \left({x}^{{log}_{\mathrm{4}} \sqrt{{x}}} \right)+{log}_{\mathrm{2}} \mathrm{2}={log}_{\mathrm{2}} \left({x}^{{log}_{\mathrm{4}} {x}} \right) \\ $$$$.... \\ $$$$\frac{\mathrm{1}}{\mathrm{4}}{log}_{\mathrm{2}} {x}^{\mathrm{2}} =\frac{\mathrm{1}}{\mathrm{2}}{log}_{\mathrm{2}} {x}^{\mathrm{2}} −\mathrm{1} \\ $$$$.... \\ $$$${log}_{\mathrm{2}} {x}^{\mathrm{2}} =\mathrm{4} \\ $$$${log}_{\mathrm{2}} {x}=\mathrm{2}\:\:{and}\:\:{log}_{\mathrm{2}} {x}=−\mathrm{2} \\ $$$${x}=\mathrm{4}\:\:{or}\:{x}=\frac{\mathrm{1}}{\mathrm{4}} \\ $$

Commented by MJS last updated on 21/Nov/19

funny you get the solution...  but  log_b  (p+q) ≠log_b  p +log_b  q  b^(log_b  (p+q)) =p+a but b^(log_b  p +log_b  q) =pq  so your 2^(nd)  line is not equivalent to the first  line

$$\mathrm{funny}\:\mathrm{you}\:\mathrm{get}\:\mathrm{the}\:\mathrm{solution}... \\ $$$$\mathrm{but} \\ $$$$\mathrm{log}_{{b}} \:\left({p}+{q}\right)\:\neq\mathrm{log}_{{b}} \:{p}\:+\mathrm{log}_{{b}} \:{q} \\ $$$${b}^{\mathrm{log}_{{b}} \:\left({p}+{q}\right)} ={p}+{a}\:\mathrm{but}\:{b}^{\mathrm{log}_{{b}} \:{p}\:+\mathrm{log}_{{b}} \:{q}} ={pq} \\ $$$$\mathrm{so}\:\mathrm{your}\:\mathrm{2}^{\mathrm{nd}} \:\mathrm{line}\:\mathrm{is}\:\mathrm{not}\:\mathrm{equivalent}\:\mathrm{to}\:\mathrm{the}\:\mathrm{first} \\ $$$$\mathrm{line} \\ $$

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