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JEE Advanced 2013 · Complex Numbers and Quadratic Equations

If 3x=4x−13^{x} = 4^{x-1}, then x=x =

Answers: A. 2log⁡322log⁡32−1\frac{2\log_{3}2}{2\log_{3}2-1} · B. 22−log⁡23\frac{2}{2-\log_{2}3} · C. 11−log⁡43\frac{1}{1-\log_{4}3}

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Written solution

If 3^{x} = 4^{x-1}, then x =

3x=4x−13^{x} = 4^{x-1}

To find: x from 3x=4x−1\text{To find: } x \text{ from } 3^{x} = 4^{x-1}

1. take log⁡3 on both sides1.\ \text{take } \log_3 \text{ on both sides}

2. log⁡34=2log⁡322.\ \log_3 4 = 2\log_3 2

3. solve the linear equation for x3.\ \text{solve the linear equation for } x

y=3xy = 3^x

y=4x−1=4x4y = 4^{x-1} = \dfrac{4^x}{4}

log⁡3(3x)=log⁡3(4x−1)\log_3(3^x) = \log_3(4^{x-1})

x=(x−1)log⁡34x = (x-1)\log_3 4

log⁡34=log⁡322=2log⁡32\log_3 4 = \log_3 2^2 = 2\log_3 2

x=2(x−1)log⁡32x = 2(x-1)\log_3 2

x=2xlog⁡32−2log⁡32x = 2x\log_3 2 - 2\log_3 2

x−2xlog⁡32=−2log⁡32x - 2x\log_3 2 = -2\log_3 2

x(1−2log⁡32)=−2log⁡32x(1 - 2\log_3 2) = -2\log_3 2

x=−2log⁡321−2log⁡32x = \dfrac{-2\log_3 2}{1 - 2\log_3 2}

x=2log⁡322log⁡32−1x = \dfrac{2\log_3 2}{2\log_3 2 - 1}

x≈4.82:3x≈199,4x−1≈199x \approx 4.82:\quad 3^{x} \approx 199,\quad 4^{x-1} \approx 199

2log⁡322log⁡32−1=22−log⁡23\dfrac{2\log_3 2}{2\log_3 2 - 1} = \dfrac{2}{2-\log_2 3}

=11−log⁡43= \dfrac{1}{1-\log_4 3}

x=2log⁡322log⁡32−1\boxed{x = \dfrac{2\log_3 2}{2\log_3 2 - 1}}

option (1)\text{option (1)}

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