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Solve log base 2 of x, plus log base 2 of (x minus 2), equal to 3.
AThe value -2 only
BBoth 4 and -2
CThe value 4 only
DNo value at all
Answer & Solution
Correct answer: C. The value 4 only
1. Condense the left side with the product rule into one logarithm.
2. That gives log base 2 of the product x times the bracket (x minus 2), equal to 3.
3. Rewrite in exponential form, so x times the bracket (x minus 2) equals 2 cubed, which is 8.
4. Expanding gives x squared minus 2x equal to 8.
5. Writing that in standard form gives x squared minus 2x minus 8 equal to 0.
6. Factoring gives the brackets (x minus 4) and (x plus 2), so the candidates are 4 and negative 2.
7. Check each one, since a logarithm accepts only positive arguments.
8. At x equal to 4 the arguments are 4 and 2, both positive, and log base 2 of 4 plus log base 2 of 2 is 2 plus 1, which is 3.
9. At x equal to negative 2 the first argument is negative, so that candidate has to be thrown out.
10. So the value 4 only survives the check, and keeping both candidates or rejecting both would be wrong.
_Source: OpenStax Intermediate Algebra (CC BY 4.0), section 10.5 Solve Exponential and Logarithmic Equations_