Question 156·Hard·Nonlinear Functions
The polynomial function is defined by
The graph of is translated unit to the right and units up to create the graph of .
Which of the following is the least positive -intercept of the graph of ?
For translation questions, first write the new function explicitly using transformation rules: a shift right by replaces with , and a shift up by adds to the function. Then, to find x-intercepts, set the new function equal to 0. If you get a quartic with small integer coefficients, try factoring it into two quadratics by matching the constant term and middle coefficients, then use the quadratic formula on each factor. Finally, list the roots, select the positive ones, and compare them to answer the “least” or “greatest” requirement without unnecessary decimal approximations.
Hints
Connect the translation to an equation
How do you change algebraically if the graph is moved 1 unit to the right and 5 units up? Think about what happens to inside the function and to the overall output.
Relate x-intercepts to the function equation
Once you have an expression for , what equation must you solve to find the -intercepts? Remember what equals at an -intercept.
Handle the quartic equation
After simplifying , you will get a 4th-degree polynomial. Try factoring it as a product of two quadratics by matching coefficients, especially the constant term 5.
Compare the roots
After solving the two quadratics with the quadratic formula, you will get four roots. Which of these are positive, and which of the positive ones is the smallest?
Desmos Guide
Enter p(x) and build q(x) from the translation
In Desmos, type p(x) = x^4 - 8x^2 + 7. Then define the translated function using the rule for shifts: type q(x) = p(x - 1) + 5.
Find the x-intercepts of q(x)
Click on the graph of q(x) where it crosses the x-axis; Desmos will show the x-coordinates of the intercepts. Identify all positive x-intercepts and then determine which of those positive values is the smallest.
Step-by-step Explanation
Write the equation for the translated function
A translation 1 unit right replaces with inside the function, and a translation 5 units up adds 5 to the output.
So the new function is
Since , we have
Simplify q(x) and set it equal to 0
First expand and :
Now substitute into :
To find the -intercepts, set :
Factor the quartic into two quadratics
Try factoring the quartic as a product of two quadratics:
Matching the constant term and the term suggests trying integer pairs whose product is 5. With some trial, we find
So the equation becomes
Solve the quadratics for the x-intercepts
Set each factor equal to zero and solve:
- For :
- For :
Now simplify the square roots:
- , so the first quadratic gives .
- , so the second quadratic gives .
The positive -intercepts are and ; among these, the least positive value is , which matches answer choice D.