Melody

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Melody  11 feb 2022
 #1
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+3
10 jul 2020
 #8
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+1

\((2 \times 3^x)^3 +(9^x - 3)^3 = (9^x + 2 \times 3^x - 3)^3\\ let\;\; y=3^x\\ (2y)^3 +(y^2 - 3)^3 = (y^2 + 2y - 3)^3\\ LHS=y^6-9y^4+8y^3+27y-27\\ RHS=y^6+6y^5+3y^4-28y^3-9y^2+54y-27\\ so\\ -9y^4+8y^3+27y=6y^5+3y^4-28y^3-9y^2+54y\\ \color{red}y=0\;\;\text{could lead to one solution}\qquad \color{black}or\\ -9y^3+8y^2+27=6y^4+3y^3-28y^2-9y+54\\ 6y^4+12y^3-36y^2-36y+54=0\\ y^4+2y^3-6y^2-6y+9=0 \)

 

\(consider\\ f(y)=y^4+2y^3-6y^2-6y+9\\ \text{Any integer roots will be factors of 9 so they could be}\pm1,\;\;\pm3,\;\;\pm9\\ f(1)=0\;\;\text{therefore it is divisible by }(y-1)\\ f(-3)=0\;\;\text{therefore it is divisible by }(y+3)\\ \text{by division I found another factor to be}\;\; (y^2-3)\\ so\\ f(y)=(y-1)(y+3)(y^2-3)\\ (y-1)(y+3)(y-\sqrt3)(y+\sqrt3)=0\\ \text{So I have}\\ y=0,1,-3,\sqrt3, -\sqrt3\\~\\ y=3^x\qquad x=log_3y\qquad \text{y has to be greater than 0 so}\\ y=1,\sqrt3\\ x=0\;\;or\;\;0.5 \)

 

 

 

Latex:

(2 \times 3^x)^3 +(9^x - 3)^3 = (9^x + 2 \times 3^x - 3)^3\\
let\;\; y=3^x\\
(2y)^3 +(y^2 - 3)^3 = (y^2 + 2y - 3)^3\\
LHS=y^6-9y^4+8y^3+27y-27\\
RHS=y^6+6y^5+3y^4-28y^3-9y^2+54y-27\\
so\\  -9y^4+8y^3+27y=6y^5+3y^4-28y^3-9y^2+54y\\
\color{red}y=0\;\;\text{could lead to one solution}\qquad \color{black}or\\
-9y^3+8y^2+27=6y^4+3y^3-28y^2-9y+54\\
6y^4+12y^3-36y^2-36y+54=0\\
y^4+2y^3-6y^2-6y+9=0

 

consider\\
f(y)=y^4+2y^3-6y^2-6y+9\\
\text{Any integer roots will be factors of 9 so they could be}\pm1,\;\;\pm3,\;\;\pm9\\
f(1)=0\;\;\text{therefore it is divisible by }(y-1)\\
f(-3)=0\;\;\text{therefore it is divisible by }(y+3)\\
\text{by division I found another factor to be}\;\; (y^2-3)\\
so\\
f(y)=(y-1)(y+3)(y^2-3)\\
(y-1)(y+3)(y-\sqrt3)(y+\sqrt3)=0\\
\text{So I have}\\
y=0,1,-3,\sqrt3, -\sqrt3\\~\\
y=3^x\qquad x=log_3y\qquad \text{y has to be greater than 0  so}\\
y=1,\sqrt3\\
x=0\;\;or\;\;0.5

8 jul 2020
 #2
avatar+118609 
+2
5 jul 2020