Exponential Equations: Exponential Growth and Decay Application
The formula for exponential growth and decay is:
EXPONENTIAL GROWTH AND DECAY FORMULA
y = abx
Where a ≠ 0, the base b ≠ 1 and x is any real number
In this function, a represents the starting value such as the starting population or the starting dosage level.
The variable b represents the growth or decay factor. If b > 1 the function represents exponential growth. If 0 < b < 1 the function represents exponential decay.
When given a percentage of growth or decay, determined the growth/decay factor by adding or subtracting the percent, as a decimal, from 1.
In general if r represents the growth or decay factor as a decimal then:
b = 1 + r Growth Factor
A decay of 20% is a decay factor of 1 - 0.20 = 0. 80
A growth of 13% is a growth factor of 1 + 0.13 = 1.13
The variable x represents the number of times the growth/decay factor is multiplied.
Let's solve a few exponential growth and decay problems.
The population of Gilbert Corners at the beginning of 2001 was 12,546. If the population grew 15% each year, what was the population at the beginning of 2015?
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Step 1: Identify the known variables. Remember that the decay/growth rate must be in decimal form. Since the population is said to be growing, the growth factor is b = 1 + r. |
y = ? Population 2015 a = 12,546 Starting value r = 0.15 Decimal form b = 1 + 0.15 Growth Factor x = 2015 - 2001 = 14 Years |
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Step 2: Substitute the known values. |
y = abx y = 12,546(1.15)14 |
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Step 3: Solve for y. |
y = 88,772 |
Example 1: The half-life of radioactive carbon 14 is 5730 years. How much of a 16 gram sample will remaining after 500 years?
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Step 1: Identify the known variables. Remember that the decay/growth rate must be in decimal form. A half-life, the amount of time it takes to deplete half the original amount, infers decay. In this case b will be a decay factor. The decay factor is b = 1 - r. In this situation x is the number of half-lives. If one half-life is 5730 years then the number of half-lives after 500 years is |
y = ? Remaining grams a = 16 Starting value r = 50% = 0.5 Decimal form b = 1 - 0.5 Decay Factor No. of Half lives |
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Step 2: Substitute the known values. |
y = abx
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Step 3: Solve for y. |
y = 15.1 grams |
Example 2: A patient is given a 300 mg dose of medicine that degrades by 25% every hour. What is the remaining drug concentration after a day?
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Step 1: Identify the known variables. Remember that the decay/growth rate must be in decimal form. A drug degrading infers decay. In this case b will be a decay factor. The decay factor is b = 1 - r. In this situation xis the number of hours, since the drug degrades at 25% per hour. There are 24 hours in a day. |
y = ? Remaining drug a = 300 Starting value r = 0.25 Decimal form b = 1 - 0.25 Decay Factor x = 24 Time |
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Step 2: Substitute the known values. |
y = abx y = 300(0.75)24 |
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Step 3: Solve for y. |
0 = 0.30 mg |
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Related Links: Math algebra Exponential Equations: Introduction and Simple Equations Exponential Equations: Simple Equations with the Natural Base Algebra Topics Exponential Functions |
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