Logarithms
posted by Anonymous .
Solve log4 x + log4(x2)=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(x2)=log4(15)
log4 [x + log4(x2)]=log4(15)
(x^2 + x)=4
x^2 + x4=0

I see two major errors
1. from log A + log B = log (A x B)
your second line should have been
log4 [x(x2)]=log4(15)
then in your third line
the right side should have been 15, not 4
so
x(x2) = 15
x^2  2x  15 = 0
(x5)(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 
should have been
x5)(x+3) = 0
x = 5 or x = 3, but x=3 does not work since you cannot take the log of a negative
so x =l 5 is still the only solution