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How do I solve this log equation via algebra?

log (X+4) - log (X) = log (X+2)

i wrote

log (X+4)/(X) = log (X+2) and canceled the logs but the book gives some weird answer with square roots. any suggestions pls?

3 Answers

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  • AbCad
    Lv 5
    1 decade ago
    Favorite Answer

    You started out right, but you have to continue solving for x after.

    log (x + 4) - log (x) = log (x + 2)

    log ((x + 4)/x) = log (x + 2)

    (x + 4)/x = x + 2

    Multiply both sides by x.

    x + 4 = x^2 +2x

    Subtract x and 4 from both sides.

    x^2 + x - 4 = 0

    Quadratic formula.

    [-b +or- root(b^2 - 4ac)] /2

    [-1 +or- root(1 - -16)] / 2

    [-1 +or- root(17)] / 2

    You can remove the "minus" option because if you did it that way, x would be negative, and that goes against log domains.

    So, your answer is:

    [-1 + root(17)] / 2

    hope i helped :)

  • 1 decade ago

    well if its just log of base 10 then u just get rid of all the logs so itll be (x+4)/x=x+2.

  • Anonymous
    1 decade ago

    (x+4)/x = x+2 --->x+4 = x^2 + 2x --->x^2+x-4 = 0 -->x = (-1 + sqrt17)/2

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