Question 135·Hard·Nonlinear Functions
The function is defined by . A new function is defined by
where is a nonzero constant. If the leading coefficient of is , the graph of in the -plane has -intercepts at , , and , where , , and are distinct constants. What is the value of ?
For composite-function root problems like this, focus on where the function equals zero, not on vertical stretch factors. First, factor the base function to find its zeros. Then, handle the input transformation (here, x → 2 − x) by setting 2 − x equal to each original zero and solving for x. Finally, use those transformed roots to answer what the problem actually asks for (here, the sum p + q + r), and avoid getting distracted by constants like k that do not affect the x-intercepts.
Hints
Connect x-intercepts to an equation
The x-intercepts of the graph of a function occur where its output is zero. For this problem, what equation involving g(x) should you set up, and how does that relate to f(2 − x) when k is nonzero?
Work with f(x) first
Before dealing with f(2 − x), factor f(x) = x^3 − 4x. Once it is factored, you can easily read off the values of x that make f(x) = 0.
Apply the transformation 2 − x
The equation f(2 − x) = 0 means that the input to f is 2 − x. Take each zero of f (each value that makes f equal 0) and set 2 − x equal to that value, then solve for x. Those x-values are the x-intercepts of g(x).
Finish with the requested sum
After you find all three distinct x-intercepts of g(x), remember that the question asks for p + q + r, not the individual values. Add the three x-values you found.
Desmos Guide
Graph the original function f(x)
In Desmos, enter f(x) = x^3 - 4x to see the base cubic and where it crosses the x-axis. This helps you identify the original zeros of f(x).
Graph the transformed function without the constant k
Enter g(x) = f(2 - x). Since multiplying by a nonzero constant k only stretches the graph vertically, it does not change the x-intercepts, so this version has the same intercepts as the full g(x).
Identify and add the x-intercepts
On the graph of g(x) = f(2 - x), click on each point where the curve crosses the x-axis to read off the three x-coordinates. Then, add those three x-values together to get p + q + r.
Step-by-step Explanation
Relate x-intercepts of g to an equation
The x-intercepts of the graph of occur where .
Given
with , the equation is equivalent to
Because is nonzero, this happens exactly when
So to find the x-intercepts of , we just need to solve . The value of (and the fact that the leading coefficient is 12) only stretches the graph vertically and does not change the x-intercepts.
Find the zeros of f(x)
First, factor :
From this factorization, when any factor is zero:
So the zeros of are , , and .
Use the transformation 2 − x to get the zeros of f(2 − x)
Let . Then is the same as , so must be one of the zeros of :
Now solve for each of these values of :
- If :
- If :
- If :
Thus, when , , or . These are the x-intercepts of , so , , and are , , and (in some order).
Compute p + q + r
We now add the three distinct x-intercepts:
So the value of is . (Choice D)