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How to Put Log Base in Calculator Ti-84 Plus

Reviewed by Calculator Editorial Team

When solving math problems, you often need to calculate logarithms with different bases. The TI-84 Plus calculator can handle this, but the process isn't immediately obvious. This guide explains how to input and calculate logarithms with custom bases on your TI-84 Plus.

Introduction

The TI-84 Plus is a powerful scientific calculator that can perform logarithmic calculations. By default, it calculates base-10 logarithms (log₁₀), but you can also calculate logarithms with other bases using the change of base formula.

This guide will walk you through:

  • Using the basic logarithm function
  • Changing the logarithm base
  • A practical example

Basic Logarithm Function

To calculate a base-10 logarithm on your TI-84 Plus:

  1. Press the 2ND key
  2. Press the LOG key (this is the log₁₀ function)
  3. Enter your number
  4. Press the = key to get the result

Formula: log₁₀(x) = log(x) / log(10)

For example, to calculate log₁₀(100):

  1. Press 2ND LOG 100 =
  2. The calculator will display 2

Changing the Logarithm Base

To calculate logarithms with other bases, use the change of base formula:

Change of base formula: logₐ(b) = logₐ(b) = log(b) / log(a)

On your TI-84 Plus, you can implement this formula by:

  1. Calculating the numerator: Press 2ND LOG and enter the number (b)
  2. Calculating the denominator: Press 2ND LOG and enter the base (a)
  3. Divide the numerator by the denominator

Tip: Remember that logₐ(b) is only defined when a > 0, a ≠ 1, and b > 0.

Worked Example

Let's calculate log₂(8):

  1. Press 2ND LOG 8 = to get the numerator (approximately 0.9031)
  2. Press 2ND LOG 2 = to get the denominator (approximately 0.3010)
  3. Divide the numerator by the denominator: 0.9031 / 0.3010 ≈ 3

The result is 3, which makes sense because 2³ = 8.

FAQ

Can I calculate natural logarithms (ln) on the TI-84 Plus?
Yes, press 2ND LN to calculate natural logarithms (base e).
What if I get an "ERROR" when calculating logarithms?
This usually means you tried to calculate logₐ(b) where a ≤ 0, a = 1, or b ≤ 0. Check your inputs and try again.
Is there a way to store logarithm calculations for later use?
Yes, you can store results in variables. For example, to store log₂(8) in variable A, enter 2ND LOG 8 / (2ND LOG 2) → STO→ A.