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Logarithmic Function Context and Data Modeling

Unit 2 · Exponential and Logarithmic Functions

What AP Precalc asks here

Logarithmic models compress quantities that range over many orders of magnitude. Standard log scales include pH (log10[H+]-\log_{10}[\text{H}^+]), Richter (log10(A/A0)\log_{10}(A/A_0)), and decibels (10log10(P/P0)10 \log_{10}(P/P_0)). Reading these scales additively is the most common mistake: a difference of 1 unit corresponds to a multiplicative ratio of 10, not 1. Log models also capture diminishing returns — output grows but ever more slowly.

Common log scales

pH
pH=log10[H+]\text{pH} = -\log_{10}[\text{H}^+]
Richter magnitude
M=log10 ⁣(AA0)M = \log_{10}\!\left(\tfrac{A}{A_0}\right)
Decibel
dB=10log10 ⁣(PP0)\text{dB} = 10 \log_{10}\!\left(\tfrac{P}{P_0}\right)
Caution: A 1-unit log10\log_{10} difference = a factor-of-10 multiplicative change in the underlying quantity. pH 4 to pH 6 is 100× less [H+][H^+], not 2×.
Type 1

Log-scale comparison

Take the difference in scale values, then raise 10 to that power for the multiplicative ratio.

Example 1
Earthquake A has Richter magnitude 6; earthquake B has magnitude 8. How many times greater is B's amplitude? (A) About 2× greater (B) About 10× greater (C) About 100× greater (D) About 1,000× greater

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Type 2

Compute a log-scale formula value

Substitute the underlying quantities into the formula and evaluate the log.

Example 2
Sound power P1=105P_1 = 10^{-5} W/m² and reference P0=1012P_0 = 10^{-12} W/m². Compute the decibel level. (A) 77 dB (B) 5050 dB (C) 7070 dB (D) 700700 dB

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Type 3

Build and apply a log model

Use one data point to find aa and another to find bb in y=a+blog10xy = a + b \log_{10} x. Then evaluate at a new xx.

Example 3
Data: (1,0),(10,2),(100,4)(1, 0), (10, 2), (100, 4). Find a log model y=a+blog10xy = a + b \log_{10} x. (A) y=2log10xy = 2 \log_{10} x (B) y=log10x+2y = \log_{10} x + 2 (C) y=4log10xy = 4 \log_{10} x (D) y=12log10xy = \tfrac{1}{2} \log_{10} x

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Logarithmic Function Context and Data Modeling | AP Precalculus — Mathfolis