PSAT Math Grid-Ins Question 48: Answer and Explanation
Question: 48
An exponential function is given in the form f(x) = a · bx. If f(0) = 3 and f(1) = 15, what is the value of f(- 2)?
Correct Answer: 3/25 or .12
Explanation:
3/25 or .12
Difficulty: Hard
Category: Passport to Advanced Math/Functions
Getting to the Answer: When a question involving a function provides one or more ordered pairs, substitute them into the function to see what information you can glean. Start with x = 0 because doing so often results in the elimination of a variable:
Now you know the value of a, so the equation looks like f(x) = 3 · bx. Substitute the second pair of values into the new equation:
The exponential function is f(x) = 3 · 5 x. The final step is to find the value being asked for, f(- 2). Substitute - 2 for x and simplify:
Grid this in as 3/25 or .12.
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