The TI-83 Plus, TI-84 Plus, and TI-89 all have the ability to calculate logarithms, reached by pressing the ln( or log( buttons on the TI-83 Plus and TI-84 Plus, or the button on the TI-89. In these cases, “ln” corresponds to the natural log, or logarithm base e, where e is Euler’s number, approximately 2.718282. However, for many problems it is necessary to evaluate logarithms with different bases. The TI-83 Plus and TI-84 Plus also make it easy to evaluate logarithms base 10 with the button, but beyond this, there is not a built in feature for evaluating logarithms of other useful bases.
However, there is a simple trick to remember which helps avoid this. For any base, the following “change of base” identity holds:
In the more specific case, we see that we don’t even need a logarithm button, since we can evaluate any logarithm with the ln( function.
For example, to evaluate the logarithm base 2 of 8, enter ln(8)/ln(2) into your calculator and press ENTER. You should get 3 as your answer. Try it for yourself!
