A subscript is used to indicate the base of a logarithm. For instance, common logs have a base of 10 while natural logs have a base of e. Unfortunately, the TI-83 graphing calculator does not support subscripts. That, however, does not mean you cannot solve such a log. The key lies in using the "Change of Base Property" to turn such a log into a natural or common log.
Write your log base a of b as common log b divided by common log a: log_a(b) = log(b)/log(a). For example, log_2(100) would turn into log(100)/log(2).
Submit the first log by pressing the "LOG" button, entering the value of b, pressing the right parenthesis key and pressing the division key.
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Finish the calculation by pressing the "LOG" button, entering the value of a, pressing the right parenthesis key and pressing the "ENTER" button.