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Mathematics

Solve for x: log3(x + 1) - 1 = 3 + log3(x - 1)

Logarithms

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Answer

Given,

log3(x + 1) - 1 = 3 + log3(x - 1)

Since log33 = 1, the above equation can be written as

⇒ log3(x + 1) - log33 = 3log33 + log3(x - 1)

⇒ log3(x + 1) - log33 = log333 + log3(x - 1)

log3(x+1)3=log3 [33×(x1)]\text{log}3\dfrac{(x + 1)}{3} = \text{log}3\space[3^3 \times (x - 1)]

log3(x+1)3=log3 [27(x1)]\text{log}3\dfrac{(x + 1)}{3} = \text{log}3\space[27(x - 1)]

x+13=27(x1)\dfrac{x + 1}{3} = 27(x - 1)

⇒ x + 1 = 81(x - 1)

⇒ x + 1 = 81x - 81

⇒ 81x - x = 1 + 81

⇒ 80x = 82

⇒ x = 8280=4140=1140.\dfrac{82}{80} = \dfrac{41}{40} = 1\dfrac{1}{40}.

Hence, x = 11401\dfrac{1}{40}.

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