Prove that the equation 4^x=8x+1 has only one solution

alistianyStu

alistianyStu

Answered question

2022-12-02

Prove that the equation 4 x = 8 x + 1 has only one solution

Answer & Explanation

Nicholas Lara

Nicholas Lara

Beginner2022-12-03Added 10 answers

Try to reason by absurd. There are 2 solutions (0). Consider the continuous and differentiable function
f ( x ) = 4 x ( 8 x + 1 )
denote as x 1 and x 2 ( x 1 and x 2 [ 2 ; 3 ]) the two solutions of the equation 4 x = 8 x + 1. Apply Rolle's Theorem with regard to f(x) in the interval ] x 1 ; x 2 [ and obtain that there is a point c in ] x 1 ; x 2 [ such that f′(c)=0. But
f ( x ) = D [ 4 x ( 8 x + 1 ) ] = 4 x ln ( 4 ) 8 = 0 x = log 4 ( 8 ln 4 ) 1.26
which is not in [2;3]-> absurd conclusion, there is only 1 unique solution in [2;3].

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