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Overkill
Overkill asked in Science & MathematicsMathematics · 10 years ago

Another problem about logs?

How do you know which property to use first? Here is my problem Log X + Log (X-4)= Log 21 Im trying to solve for X.

3 Answers

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  • ?
    Lv 6
    10 years ago
    Favorite Answer

    Log X + Log (X-4)= Log 21

    Log[X(X-4)]= Log 21

    X(X-4) = 21

    X^2 - 4X -21 = 0

    Now I think you can solve this.

  • ?
    Lv 7
    10 years ago

    refresh:

    loga + logb = log(ab)

    logx + log(x - 4) = log21

    log[x(x - 4)] = log21

    log is 1 to 1

    so

    x(x - 4) = 21

    solve it you will have x = -3 and x = 7. Discard x = -3 because you can not take log of a negative number.

    so the answer is x = 7

  • Trinh Nam
    Lv 4
    10 years ago

    you need adding fist

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

    then log(x^2-4x)=log21 then

    x^2-4x=21

    x^2-4x+4=21+4

    (x-2)^2=25

    (x-2+5)(x+2-5)

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