Domain and Range Relationships: CCSS.Math.Content.HSF-IF.A.1 - Common Core: High School - Functions

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What is the range of function ?

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This question is testing the concept and understanding of a function's range. It is important to recall that range can be identified graphically or algebraically. Graphically, range contains the y-values that span the image of the function where as domain, contains the x-values of the function. Algebraically, range is known as the output y of a function when input x, is used. In other words, the input values when placed into the function results in the y values creating an (x, y) pair.

For the purpose of Common Core Standards, domain and range fall within the Cluster A of the function and use of function notation concept (CCSS.Math.content.HSF-IF.A).

Knowing the standard and the concept for which it relates to, we can now do the step-by-step process to solve the problem in question.

Step 1. Identify the function and what the question is asking.

Find the range of the function.

Step 2. Discuss the options to solve the problem.

I. Graphically plot the function by computer/technology resource. Then interpret the graph.

II. Create a table of (x, y) pairs and plot the points to create the graph. Then interpret the graph.

III. Algebraically find the vertex of the function and using the properties of polynomials determine the range.

For this particular function let's use the third option to find the range.

Solving the quadratic equation to find the range algebraically requires the understanding that the vertex of a parabola represents either the peak (maximum) of a function or the valley (minimum) of the function.

Recall that a quadratic function can be written in the form,

and the formula to find the vertex x value is,

.

Finally to find the y value of the vertex we will substitute the x value found above into the original function. If the quadratic has a positive , then the vertex will occur at the valley (minimum) of the function. If is negative then the parabola opens down thus creating a peak (maximum) of the function.

Step 3: Use algebraic technique to solve the problem.

First, identify the values of the variables.

Next, plug the values into the formula to find the vertex.

Now, substitute the x value found into the function to find the y output value.

Step 4: Interpret the solution to answer the question.

This question is asking for the range of the function. Using the formula to find the vertex and the properties of quadratics, it was discovered that the vertex is the valley of the function. This means that all other outputs will be greater than the vertex. Therefore, the range will be all real values of y for which y is greater than or equal to negative four. In mathematical terms this is represented by the following.