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Reviewed CSCA Mathematics question · Standard

If log_3 (x) + log_3 (x + 6) = 3, what is the value of x?

  1. 3
  2. 9
  3. 27
  4. 81
Show the answer and explanation

Correct answer

A. 3

Principle or equation

Use logarithm product rule: log_a M + log_a N = log_a (MN). Convert to exponential form.

Why this answer is correct

log_3 [x(x+6)] = 3 => x(x+6) = 3^3 = 27 => x^2 + 6x - 27 = 0 => (x+9)(x-3)=0 => x = -9 or 3. Since log argument must be positive, x = 3.

Example

Solve log_2 (x) + log_2 (x+2) = 3: x(x+2)=8 => x=2.

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