Posted by **Anonymous** on Monday, August 25, 2008 at 6:25pm.

Solve log4 x + log4(x-2)=log4(15)

I know how do the example in my book but I don't know what I'm doing wrong here.

log4 x + log4(x-2)=log4(15)

log4 [x + log4(x-2)]=log4(15)

(x^2 + x)=4

x^2 + x-4=0

- Logarithms -
**Reiny**, Monday, August 25, 2008 at 6:43pm
I see two major errors

1. from log A + log B = log (A x B)

your second line should have been

log4 [x(x-2)]=log4(15)

then in your third line

the right side should have been 15, not 4

so

x(x-2) = 15

x^2 - 2x - 15 = 0

(x-5)(x+2) = 0

x = 5 or x = -2, but x+-2 does not work since you cannot take the log of a negative

so x=5

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