Exponential + Logarithm Equation

Algebra Level 5

3 x 8 x x + 2 = 6 \Large 3^x\cdot8^{\frac{x}{x+2}}=6

If the sum of all real solutions can be represented by a b log c d -a-b\log_cd when a , b , c a,b,c and d d are positive integers and b > 1 b>1 . Find the minimum value of a + b + c + d a+b+c+d .


The answer is 8.

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1 solution

Rishabh Jain
Mar 25, 2016

3 x 2 3 x x + 2 = 2 3 \large 3^x \cdot 2^{\frac{3x}{x+2}}=2\cdot 3 2 3 x x + 2 2 = 3 3 x \large \implies \dfrac{2^{\frac{3x}{x+2}}}{2}=\dfrac{ 3}{3^x} 2 2 ( x 1 ) x + 1 = 3 1 x \large \implies 2^{\frac{2(x-1)}{x+1}}=3^{1-x} Taking ln \ln on both sides: ( x 1 ) x + 2 ln 2 2 = ( x 1 ) ln 3 \dfrac{\cancel{(x-1)}}{x+2}\ln 2^2=-\cancel{(x-1)}\ln 3 Here we can clearly see x = 1 \color{#3D99F6}{\boxed{x=1}} is one solution. x + 2 = ln 4 ln 3 = l o g 3 4 \large \implies x+2=-\dfrac{\ln 4}{\ln 3}=-log_3 4 x = 2 2 log 3 2 \large\implies \color{#3D99F6}{ \boxed{x=-2-2\log_{3} 2}}

Therefore sum of solution= 1 + ( 2 2 log 3 2 ) = 1 2 log 3 2 1+(-2-2\log_{3} 2)=-1-2\log_3 2 1 + 2 + 3 + 2 = 8 \therefore 1+2+3+2=\huge \color{#D61F06}{\boxed 8}

Exactly the same solution....

Ankit Kumar Jain - 5 years, 2 months ago

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Great.... :-)

Rishabh Jain - 5 years, 2 months ago

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