Brain-racking equation

Algebra Level 2

Determine x x , such that 3 x + 3 x = 1 3^x+3^{-x}=1 .

Imaginary 1 3

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1 solution

There is no such real number that satisfies the condition.

Why it’s impossible in reals? \textbf{Why it's impossible in reals?}

  • 3 x > 0 3x>0 for any real x R ∀x∈R∀
  • a + 1 a 2 a+\frac{1}{a}≥2 for any positive real a R ∀a∈R∀

Most people will fall into the trap: \textbf{Most people will fall into the trap:}

3 x + 1 3 x = 1 3^x+\frac{1}{3^x}=1

3 2 x + 1 = 3 x 3^{2x}+1=3^x

3 2 x 3 x = 1 3^{2x}-3^x=-1

Oops! Dead end. \textbf{Oops! Dead end.}

Let A = 3 x A=3^x .

A 2 A + 1 = 0 A^2-A+1=0

1 ± 1 2 4 1 1 2 1 = 0 \frac{-1±\sqrt{1^2-4\cdot1\cdot1}}{2\cdot1}=0

1 ± 3 2 = 0 \frac{-1±\sqrt{-3}}{2}=0

1 ± i 3 = 0 -1±i\sqrt{3}=0

:-P Got fooled. \textbf{:-P Got fooled.}


True, it doesn't have as a real solution. But the imaginary solution can be analytically found as:

3 x = 1 ± 3 i 2 3^x = \dfrac{1\pm\sqrt{3} i}{2}

Reexpressing R.H.S in polar notation:

3 x = e i π 3 3^x = e^{i \dfrac{\pi}{3}}

And changing L.H.S basis to e e :

3 x = e ln 3 x = e x ln 3 3^x = e^{\ln{3^x}} = e^{x\ln{3}}

Then:

x = i π 3 ln ( 3 ) \boxed{x = i \dfrac{\pi}{3\ln{(3)}}}

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