By Carl Raimer • Updated August 30, 2022

A logarithm (log) is the inverse of exponentiation. While any base can be used, the TI‑84 calculator handles base‑10 (log) and natural logarithm (ln) natively. To combine logarithms of other bases, apply the change‑of‑base formula: log_a(x)=\frac{\log(x)}{\log(a)}.

Adding Base‑10 or Natural Logarithms

Step 1

Press the LOG button (left of the 7 key). The display shows: log(.

Step 2

Enter the first number and close the parenthesis. For example, 100 gives log(100).

Step 3

Press the addition key (+) followed by LOG again, then input the second number. Example: 1000 results in log(100)+log(1000).

Step 4

Press ENTER to evaluate. In this example the answer is 3. Repeat the pattern for any number of terms.

Step 5

To work with natural logarithms, press the LN button (located beneath LOG) before entering your numbers.

Adding Logarithms of Arbitrary Bases

Step 1

Enter the first logarithm with the desired base using the change‑of‑base method. For log base 9 of 81, type LOG(81).

Step 2

Press the division key (/), producing log(81)/.

Step 3

Enter the base by pressing LOG again and typing the base value. For base 9: log(81)/log(9).

Step 4

Repeat for additional terms. Adding log base 5 of 25 yields: log(81)/log(9)+log(25)/log(5).

Step 5

Press ENTER to display the result. In this example the value is 4.

TL;DR

Write down each step to avoid mistakes. You can use LN instead of LOG in the change‑of‑base formula for the same effect.

Warnings

Always close the parenthesis after each logarithm; otherwise the calculator may interpret the expression incorrectly. Logarithms are undefined for zero or negative numbers.