Lesson 6

The purpose of this Math Talk is to elicit strategies and understandings students have for solving equations in one variable and thinking about the graph of an associated function. The connection between the graph of \(y = x^2\) and solutions to quadratic equations should be familiar from earlier courses. These understandings help students develop fluency and will be helpful later in this lesson when students solve equations that involve squares and square roots.

Display one problem at a time alongside the image showing the graph of \(y =x^2\) and the 4 lines. Give students quiet think time for each problem and ask them to give a signal when they have an answer and a strategy. Keep all problems displayed throughout the talk. Follow with a whole-class discussion.

Discuss the connections between the solutions of the equations and the graph of \(y=x^2\).

Although \(x^2=2\) has two solutions (\(\sqrt{2}\) and \(\text{-}\sqrt{2}\)), we don’t “take the square root” of each side of the equation, because by convention, \(\sqrt{2}\) means the positive square root of 2. If we want the negative square root, we need to write \(\text{-}\sqrt{2}\). Students will study this convention more closely in the following activity, so it does not need to be discussed in depth at this time.

The purpose of this activity is for students to consider some reasons for the convention that \(\sqrt{x}\) means the positive square root of \(x\). Students think about what it would mean if the square root function were not a function. In the synthesis, the convention that makes \(\sqrt{x}\) a function is introduced. It should be emphasized that there are still two square roots of every positive number, but the square root function only gives us one of those. Students will explore the consequences of this convention in future activities.

Arrange students in groups of 2. Display the following for all to see:

\(\sqrt{4} + \sqrt{9} =\) ?

Tell students that in this activity, they will think about what the answer to this question is. Give students 2 minutes to read the statement, think, and write down their answers individually, and another 2 minutes for pairs to share their thoughts. Follow with a whole-class discussion.

Invite students to share their answers and reasons. Here are some questions for discussion if needed:

• If only one of these numbers is the sum of \(\sqrt{4}\) and \(\sqrt{9}\), does that mean that 4 and 9 each have only one square root?
• If all four of these are the sum of \(\sqrt{4}\) and \(\sqrt{9}\), what would happen if we added \(\sqrt{16}\) to them? How many answers would we get?
• If more than one of these numbers is the same as \(\sqrt{4} + \sqrt{9}\), then are they the same as each other?

The goal of the discussion is for students to consider some reasons why we might want the operation of taking the square root to give us only one number. These questions do not have to be explored in depth at this time.

To conclude the discussion and preview the work ahead, display the following graphs and the functions they represent for all to see:

\(b = \sqrt{a}\)

\(d^2 = c\)

Tell students that these graphs represent the two possibilities that they have been thinking about. \(b=\sqrt{a}\) is a function, because each value of \(a\) has only one corresponding value of \(b\). But \(d^2=c\) is not a function, because many values of \(c\) have two corresponding \(d\) values. If we used a graph like \(d^2=c\) to find square roots, we would not get one unique value for each square root. This is why there is a convention in mathematics which says that the symbol \(\sqrt{x}\) means the positive square root of \(x\). All positive numbers do have two square roots, but when we write \(\sqrt{x}\), or \(x^{\frac12}\), that means only the positive square root. We indicate the negative square root by writing \(\text- \sqrt{x}\) or \(\text- x^{\frac12}\).

In this activity, students analyze the graphs of \(b=\sqrt{a}\) and \(t=s^2\) in order to think about solutions to equations like \(\sqrt{a}=4\) and \(s^2=5\). They build on the discussion from the previous activity to find that equations like \(\sqrt{a}=4\) have only one solution, while equations like \(s^2=5\) have exactly two solutions that are opposites of each other.

Since students usually see \(x\)-values on the horizontal axis and \(y\)-values on the vertical, they may look for \(a\) or \(s\) values on the wrong axis. Encourage students to annotate the graph by drawing horizontal or vertical lines that will intersect the curves at the point that represents the solution, or using some other method that is helpful for them. For example, when solving \(s^2=25\), students can draw a line representing \(t=25\) and see where it hits the graph of \(t=s^2\), since these points represent the solutions.

The purpose of this discussion is to emphasize that equations like \(\sqrt{a}=5\) have only one solution while equations like \(s^2=11\) have two solutions. Students may find this confusing since we can talk about “the square roots” (plural) of a positive number, so it is important to remind students of the discussion in the previous activity and the convention that we use the symbol \(\sqrt{a}\) for the positive square root of \(a\) while \(\text-\sqrt{a}\) is used for the negative square root of \(a\). Display the two graphs and tables from the activity for all to see throughout the discussion. Here are some questions for discussion:

• “How can you use the graph of \(b=\sqrt{a}\) to see that the equation \(\sqrt{a}=5\) has only one solution?” (The horizontal line \(b=5\) intersects the graph of \(b=\sqrt{a}\) at one point, \((25,5)\).)  
• “How can you use the graph of \(t=s^2\) to see that the equation \(s^2=12\) has two solutions? What are the exact values of the two solutions?” (The horizontal line \(t=12\) intersects the graph of \(t=s^2\) at two points, \((\text- \sqrt{12},12)\) and \((\sqrt{12},12)\). The two solutions are \(\text- \sqrt{12}\) and \(\sqrt{12}\).)