# Logs help

• Jan 26th 2009, 02:40 PM
eh501
Logs help
How do i figure these out?:

The expression log(base 2)*(x-4) is undefined for all values of x such that
(1) x>1 (2) x<or equal to 4 (3) x>0 (4) x<or equal to 0

The expression log(base 3)*(8-x) is defined for all values of x such that
(1) x>8 (2)x<8 (3)x>or equal to 8 (4)x<or equal to 8

All i need to know is how to do them, i have the answers.
• Jan 26th 2009, 04:24 PM
becker89
hello,
you just need to know the domain of a logarithmic function.
which is: base must be positive,and different from 1,
while the result of the logarithm must be greater than zero
• Feb 3rd 2009, 04:49 AM
stapel
Quote:

Originally Posted by eh501
How do i figure these out?

Logarithms are defined only for positive inputs. In particular, they are undefined for any input that is zero or less.

So, to "figure these out", take the input expression and set it "less than or equal to zero" and solve to find where the log is not defined, or set it "greater than zero" and solve to find where the log is defined.

Have fun! :D