Is it possible to solve this exponential equation algebratically, if so, how?

Calculus Level 1

1 + ln ( x ) = x \large 1 + \ln(x) = x


The answer is 1.

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1 solution

Jordan Cahn
Jan 17, 2019

1 + ln ( x ) = x ln ( x ) = x 1 x = e x 1 Exponentiate x e x 1 = 0 \begin{aligned} 1+\ln(x) &=x \\ \ln(x) &= x-1 \\ x &= e^{x-1} &&\color{#3D99F6}\text{Exponentiate} \\ x-e^{x-1} &= 0 \end{aligned} Let f ( x ) = x e x 1 f(x)=x-e^{x-1} . Then f ( x ) = 1 e x 1 f'(x) = 1-e^{x-1} . Solving f ( x ) = 0 f'(x)=0 yields x = 1 x=1 as the only critical point. Evaluating f ( 1 ) = e 0 = 1 f''(1) = -e^0 = -1 tells us that f ( 1 ) = 0 f(1)=0 is the maximum value of f ( x ) f(x) . Thus (since f ( x ) f(x) is everywhere differentiable) x = 1 x=\boxed{1} it is the only root of f ( x ) f(x) and, therefore, the only solution to our original equation.

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