base 10, and
base b other than e or 10, where 0 < b and b
1.
We consider logarithms with three different bases:
base e (natural logarithms),
base 10, and
base b other than e or 10, where 0 < b and b 1.
Recall that the domain of a logarithm function consists of all real numbers x > 0.
In the examples below, the Exact/Approx mode is in AUTO.
Base e
|
Simply use the LN key (located above the X key). For example, to calculate
ln(17), press
|
 |
Base 10
The TI-89 does not have a logarithm base 10 key. However, one can use logarithm base 10 from
the catalog. Press CATALOG to access the catalog.
To navigate to log( , press L, then 
,
then press the blue down arrow on the thumb pad, then press the blue down arrow 5 times to select
log( . Press 
. Enter the number of interest, then press the right
parenthesis.
 |
 |
For example, the screen displays the calculations for log(17). Notice that the TI-89 uses the change of base formula to convert logarithms base 10 to logarithms base e.
Other Bases
|
For logarithmic bases other than e or 10, one must apply the change of base formula to convert to base e:
|
 |
Hence, to determine log5(17), note that
log5(17) = ln(17)/ln(5). Finally press 
to get a decimal approximation. Here, log5(17) 
1.76037.
