Mathematics
Grade 9
15 min
Evaluate an exponential function
Evaluate an exponential function
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Introduction & Learning Objectives
Learning Objectives
Define an exponential function and identify its components (base, exponent, initial value).
Substitute a given input value for the variable in an exponential function.
Correctly apply the order of operations when evaluating exponential functions.
Evaluate exponential functions for positive integer, negative integer, and zero exponents.
Use a calculator to approximate the value of an exponential function for non-integer exponents.
Interpret the result of evaluating an exponential function within a real-world context.
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In this tutorial, you will learn the fundamental skill of evaluating exponential functions. This means p...
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Key Concepts & Vocabulary
TermDefinitionExample
Exponential FunctionA function where the input variable (x) appears as an exponent. The general form is f(x) = a â‹… b^x, where 'a' is the initial value and 'b' is the constant base.f(x) = 3 â‹… 2^x is an exponential function. Here, the initial value is 3 and the base is 2.
Base (b)The number that is repeatedly multiplied by itself. In f(x) = a â‹… b^x, 'b' is the base. It must be a positive number and not equal to 1.In the function g(t) = 10(1.5)^t, the base is 1.5.
Exponent (x)The variable that indicates how many times the base is to be multiplied by itself.In the function h(x) = 5^x, 'x' is the exponent. If we evaluate h(3), the exponent is 3, meaning 5 â‹… 5 â‹… 5.
Initial Value (a)The starting amount of the function, or the value of...
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Core Formulas
The Exponential Function Form
f(x) = a \cdot b^x
This is the standard form. To evaluate, substitute the given value for 'x', calculate the power b^x first, and then multiply by 'a'.
Zero Exponent Rule
b^0 = 1 \quad (for\ b \neq 0)
Any non-zero base raised to the power of zero is equal to 1. This is useful for finding the initial value of a function, f(0) = a â‹… b^0 = a â‹… 1 = a.
Negative Exponent Rule
b^{-x} = \frac{1}{b^x}
A base raised to a negative exponent is equal to the reciprocal of the base raised to the positive exponent. This rule is essential for evaluating functions for negative inputs.
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Challenging
The value of a classic car appreciates according to the function V(t) = 20000 â‹… (1.25)^t, where V is the value in dollars and t is the time in years since its restoration. What was the theoretical value of the car one year *before* it was restored (t = -1)?
A.$25,000
B.$15,000
C.$22,500
D.$16,000
Challenging
A population of frogs, F(t) = 80 â‹… (1.5)^t, and a population of fish, G(t) = 500 â‹… (0.8)^t, are modeled over t years. After 2 years, how does the frog population compare to the fish population?
A.The frog population is larger than the fish population.
B.The frog population is smaller than the fish population.
C.The populations are equal.
D.It's impossible to compare them.
Challenging
Evaluate f(4) for the function f(x) = -3 â‹… (1/2)^x.
A.3/16
B.-3/8
C.-3/16
D.24
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