Just An Equation

Algebra Level 3

Find the sum of all solutions to the equation above.

13 4 \frac{13}{4} 9 2 \frac{9}{2} 9 4 \frac{9}{4} 7 2 \frac{7}{2}

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4 solutions

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First note that x = 0 x = 0 is not a solution, since 0 0 = 0 0*\sqrt{0} = 0 but 0 0 = 0 0 0^{\sqrt{0}} = 0^{0} is indeterminate. So given that x 0 x \ne 0 , we can write this equation as

x x x 3 2 = 1 x ( x 3 2 ) = 1. \large \dfrac{x^{\sqrt{x}}}{x^{\frac{3}{2}}} = 1 \Longrightarrow x^{(\sqrt{x} - \frac{3}{2})} = 1.

Now since 1 y = 1 1^{y} = 1 for any real y y we have that x = 1 x = 1 is a solution.

For x 1 , 0 x \ne 1,0 we must have that x 3 2 = 0 x = ( 3 2 ) 2 = 9 4 . \sqrt{x} - \dfrac{3}{2} = 0 \Longrightarrow x = \left(\dfrac{3}{2}\right)^{2} = \dfrac{9}{4}.

The sum of all possible solutions is thus 1 + 9 4 = 13 4 . 1 + \dfrac{9}{4} = \boxed{\dfrac{13}{4}}.

Awesome solution!

Another way I like to think about it/present it is x x = x x = x 3 2 , x^{\sqrt{x}} = x\sqrt{x} = x^{\frac{3}{2}}, so either x = 1 x=1 or x = 3 2 . \sqrt{x} = \frac{3}{2}.

Eli Ross Staff - 5 years, 6 months ago

very elegant

Iqbal Mohammad - 5 years, 6 months ago
Pablo Padilla
Nov 21, 2015

x x = x x x^{\sqrt{x}}=x \sqrt{x} x ln x = ln ( x x ) \sqrt{x}\ln{x}=\ln{(x\sqrt{x})} x ln x ln x ln x 1 / 2 = 0 \sqrt{x}\ln{x}-\ln{x}-\ln{x^{1/2}}=0 x ln x ln x 1 2 ln x = 0 \sqrt{x}\ln{x}-\ln{x}-\frac{1}{2}\ln{x}=0 ( x 1 1 2 ) ln x = 0 (\sqrt{x}-1-\frac{1}{2})\ln{x}=0 Hence, the solutions are: ln x = 0 x = 1 \ln{x}=0 \Rightarrow x=1 or: x 3 2 = 0 x = 9 / 4 \sqrt{x}-\frac{3}{2}=0 \Rightarrow x=9/4 1 + 9 4 = 13 4 1+\frac{9}{4}=\boxed{\frac{13}{4}}

Michael Fuller
Nov 22, 2015

From observation, 1 1 is a solution, and 0 0 is not as 0 0 {0}^{0} is indeterminate. We also have:

ln ( x x ) = ln ( x x ) \ln { \left( { x }^{ \sqrt { x } } \right) } =\ln { \left( x\sqrt { x } \right) }

x ln x = ln ( x 3 2 ) = 3 2 ln x \sqrt { x } \ln { x } =\ln \left( {x}^{\large\frac{3}{2}} \right) =\dfrac{3}{2} \ln {x}

x = 3 2 \sqrt { x } =\frac { 3 }{ 2 }

x = 9 4 x=~~\frac { 9 }{ 4 }

Therefore the sum of the solutions is 1 + 9 4 = 13 4 1+\dfrac{9}{4}=\large \color{#20A900}{\boxed{\dfrac{13}{4}}} .

x x = x x x^{\sqrt{x}}=x\sqrt{x} e x l n x = e l n ( x x ) e^{\sqrt{x}lnx}=e^{ln(x\sqrt{x})} x l n x = 3 2 l n x \sqrt{x}lnx=\frac{3}{2} \cdot lnx x 1 = 1 \rightarrow x_{1}=1 x = 3 2 \sqrt{x}=\frac{3}{2} x 2 = 4 9 \rightarrow x_{2}=\frac{4}{9} x 1 + x 2 = 13 4 x_{1}+x_{2}=\frac{13}{4}

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