Question 228·Hard·Nonlinear Functions
The function is defined by
The graph of in the -plane has one local maximum and one local minimum.
Which of the following intervals contains the -coordinate of the local minimum of the graph of ?
For questions asking where a polynomial has a local maximum or minimum, use your graphing calculator. Graph the function and look for peaks (local maxima) and valleys (local minima). Click on the turning point to find its coordinates. For cubic functions with a squared factor like , the graph bounces off the x-axis at , which is often a local extremum.
Hints
Understand what local minimum means
A local minimum is the lowest point in a "valley" of the graph. Look for where the curve dips down and then rises back up.
Use your graphing calculator
Graph and look for the lowest point (the valley) on the curve. The x-coordinate of that point is what you need.
Think about the shape of the graph
This is a cubic function. The factor means the graph touches (but doesn't cross) the x-axis at . The factor means it crosses at . Where might the valley be?
Desmos Guide
Graph the function
In Desmos, type f(x) = (x+2)^2(x-5) and make sure the viewing window shows roughly from to .
Locate the local minimum visually
Look at the graph: you should see the curve touch the x-axis at (a local high point) and then dip down to a low point before rising again. Zoom in around that valley.
Find the x-coordinate of the minimum
Click on the lowest point of the curve (the valley). Desmos will show its coordinates. The x-coordinate is approximately 2.67. Compare this to the answer intervals and select the one that contains this value.
Step-by-step Explanation
Understand the function's zeros
The function has:
- A double zero at (the graph touches but doesn't cross the x-axis here)
- A simple zero at (the graph crosses the x-axis here)
Since the leading coefficient is positive (expanding gives ..), the graph goes down to on the left and up to on the right.
Identify the shape of the graph
Because the graph has a double zero at , it "bounces" off the x-axis there. This creates a local maximum at .
The graph then dips down (going negative) before rising to cross the x-axis at . The lowest point of this dip is the local minimum.
Find the local minimum using a graphing calculator
Graph in Desmos. You'll see:
- The graph touches the x-axis at (local maximum)
- The graph dips down to a valley somewhere between and
- The graph crosses the x-axis at
Click on the lowest point of the valley. Desmos shows the x-coordinate is approximately .
Match to the answer choices
The x-coordinate of the local minimum is approximately , which is .
This value lies in the interval , so the answer is C.