algebra

Solve: log x + log(x+15)=2

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  1. using the rule:
    log (AB) = logA + logB

    log x + log(x+15)=2
    log(x(x+15)) = 2
    x(x+15) = 10^2
    x^2 + 15x - 100 = 0
    (x + 20)(x-5) = 0
    x = -20 or x = 5
    BUT if x = -20 , we would have log (-20) in our expression, and log(a negative) is undefined

    so , x = 5

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