Question 150·Hard·Nonlinear Functions
For , two functions are defined as follows.
I.
II.
Which of these equations displays, as a constant or coefficient, the maximum value of the function it defines, where ?
For exponential functions on a restricted domain like , first decide whether the function is increasing or decreasing: if it involves with or with , it is decreasing, so the maximum occurs at the smallest allowed . Evaluate the function at that endpoint to get the maximum value. Then compare this value to the constants and coefficients as the equation is written, without algebraic simplification, to see if the maximum is shown directly; be especially careful with horizontal shifts in the exponent like , which can hide the true maximum value.
Hints
Think about where the maximum occurs
For each function, is it increasing or decreasing when ? Where will the maximum value happen on this domain: at smaller or larger ?
Evaluate at the key -value
Once you decide where the maximum occurs for , plug that -value into each function to find its maximum output.
Connect the maximum value to the equation's form
Compare the maximum output you found to the numbers that appear directly in the equation (constants and coefficients). Does the maximum value show up exactly as one of these numbers, or does it only appear after simplifying?
Pay close attention to the exponent shifts
In function I, the exponent is , not just . How does that extra in the exponent affect the maximum output compared with the visible coefficient 14?
Desmos Guide
Graph both functions with the domain restriction
In Desmos, enter f(x)=14*7^(-(x+1)) {x>=0} and g(x)=2*7^(-x) {x>=0} so that both functions are drawn only for .
Identify the maximum values from the graphs
Use the trace tool or create a table for each function and look at the -values near . Note the highest -value each graph attains on and compare that value to the constants or coefficients in each equation.
Compare the displayed numbers to the observed maxima
Look at the coefficient and constants in each equation (14 and the exponent in the first, 2 in the second) and decide which equation has a constant or coefficient that exactly matches the maximum -value you observed in Desmos.
Step-by-step Explanation
Understand how these exponential functions behave
Both functions involve the factor (or something very close to it). Since :
- increases as increases.
- Therefore decreases as increases.
Because and the functions are decreasing, the maximum value of each function on this domain will occur at the smallest allowed , which is .
Find the maximum value of each function on
Evaluate each function at .
For I:
For II:
Since each function is decreasing for , these values at are the maximum values of and on the given domain, and both maxima are 2.
Interpret the phrase "displays, as a constant or coefficient"
The question is not asking which function has the larger maximum. Instead, it asks:
Does the equation, as written, show the maximum value directly as one of its constants or coefficients?
So we must look at the form of each equation and see whether the number 2 (the maximum value) appears clearly as a coefficient or constant, without algebraic rewriting.
Check equation I
Equation I is
From Step 2, we know the maximum value of is 2, but in this equation:
- The visible constant multiplier is 14.
- The exponent is , which means is always less than or equal to for .
The maximum value 2 comes from both the 14 and the together; 2 does not appear directly as a standalone constant or coefficient in this form. So equation I does not display its maximum value as a constant or coefficient.
Check equation II and conclude the correct answer
Equation II is
From Step 2, the maximum value of on is 2, which is achieved at because .
In this equation, the number 2 is a clear coefficient in front of , and this coefficient is exactly the maximum value of the function on the given domain.
- Equation I does not show the maximum value directly as a constant or coefficient.
- Equation II does show the maximum value directly as a coefficient.
Therefore, the correct answer is II only.