A graph that touches the -axis gives a repeated root: its factor appears more than once. For the least possible degree, use two copies at a touch and one at each crossing. Then use a labeled point to find the number multiplying the factors, and evaluate the function in Desmos. Treating the touch like a crossing builds the wrong polynomial.
Hints
- Hint 1
An -intercept is an input where the output is . A root at gives a factor . What factors come from the three places where the curve meets the axis?
- Hint 2
At , the curve touches and turns instead of crossing. To stay on the same side of the axis, that root needs an even number of copies. What is the smallest even number?
- Hint 3
The point means . Substitute into your factored expression to find its missing multiplier before you try to calculate .
Step-by-step
Build the polynomial from the graph
Step 1Turn the intercepts into factors
The graph touches the -axis at , so its factor must repeat. A touch needs a squared factor at the least possible degree. The graph crosses at and , so use one factor for each of those roots, or inputs with output :
The number sets the graph’s vertical scale.
- Step 2
Use the labeled point
The point says input gives output . Substitute both coordinates to make an equation for the missing multiplier:
Unlike a root, this point gives an equation that can determine .
- Step 3
Find the multiplier in Desmos
Type . The tells Desmos to find from the equation. Under PARAMETERS, Desmos shows , or .
- Step 4
Define the function
Type , using the value of Desmos found. This keeps the squared factor at the touch; dropping that square would change the function.
- Step 5
Evaluate at the requested input
Type , which means put in for . Desmos displays ; tap its fraction button to see . So the output at is . Choice A.
Lessons that teach this
- SAT Nonlinear FunctionsIntermediateCoreBuild a quadratic function from conditions
- SAT Nonlinear FunctionsIntermediateCoreRead nonlinear graphs and representations
- SAT Advanced AlgebraAdvancedUse polynomial identities, factors, and unknown coefficients
- DesmosAdvancedCoreFactor, root, and remainder tests
- DesmosBeginnerRead points of interest from a graph