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Log(x + 2) – logx =log(2x – 1) - log(3x – 12)​

User MoustafaS
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1 Answer

1 vote

First note that any solution must belong to all of the intervals

x + 2 > 0 ⇒ x > -2

x > 0

2x - 1 > 0 ⇒ x > 1/2

3x - 12 > 0 ⇒ x > 4

so x must be larger than 4.

Recall that log(a) - log(b) = log(a/b). So we can rewrite

log(x + 2) - log(x) = log(2x - 1) - log(3x - 12)

log((x + 2)/x) = log((2x - 1)/(3x - 12))

Take the antilogarithm/exponential of both sides (note that the base of the logarithm doesn't matter here):

exp[log((x + 2)/x)] = exp[log((2x - 1)/(3x - 12))]

(x + 2)/x = (2x - 1)/(3x - 12)

Eliminate the fractions:

x (3x - 12) • (x + 2)/x = x (3x - 12) • (2x - 1)/(3x - 12)

(3x - 12) (x + 2) = x (2x - 1)

Expand both sides, collect terms and factorize to solve for x :

3x² - 6x - 24 = 2x² - x

x² - 5x - 24 = 0

(x - 8) (x + 3) = 0

x - 8 = 0 or x + 3 = 0

x = 8 or x = -3

User Deeptechtons
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