Home
Episode 5 Supports
Episode Description
Repeating Your Reasoning: Sasha and Keoni find the x-value of a point on the parabola with a y-value of 10.
Focus Questions
For use in a classroom, pause the video and ask these questions:
1. [Pause video at 1:06]. Keoni labeled one side of the triangle 9 and another side 11. Where are those values coming from? Does he have to count?
2. [Pause video at 2:56]. Keoni wants to know what 6.322 is. Sasha exclaims, “say it’s 40.” What do you think her reasoning is?
Supporting Dialogue
Invite your students to consider the varied student work in the room:
- As students work, you can walk around the room to examine the kinds of methods of solving the problem that are being used. When you find two different ways that the students are expressing their work, asks the students if they would be willing to share their work with the class or allow you to share it with the class.
- As students, or you, share their work, ask a student in the classroom to restate and compare what they heard. For example: “Can someone restate William’s methods?” and “What is different about the methods?”
- As students work, you can walk around the room to examine the kinds of methods of solving the problem that are being used. When you find two different ways that the students are expressing their work, asks the students if they would be willing to share their work with the class or allow you to share it with the class.
Math Extensions
1. Find the x-value of a point on the parabola when the y-value is ½.
2. Find the x- value of a point on the parabola when the y-value is √30.
Mathematics in this Lesson
Lesson Description
Targeted Understanding
CC Math Standards
CC Math Practices
Lesson Description
Keoni and Sasha work with a parabola on the coordinate grid. They use the properties of the grid and the Pythagorean theorem to determine if the coordinates of a point are on a given parabola. They apply these methods to find the missing x-value of a point on the parabola for a given y-value.
Targeted Understandings
This lesson can help students:
- Identify and apply key elements of the geometric definition of a parabola in the context of the Cartesian coordinate grid.
- Conceive of a point on a Cartesian coordinate grid, not only as a location, but also as representing distances in 2-dimensional space.
- Apply the Pythagorean Theorem in a new setting to measure distance.
Common Core Math Standards
• CCSS.M.HSG.GPE.A.2: Derive the equation of a parabola given a focus and directrix.
Lesson 2 connects the geometric definition of a parabola from Lesson 1 with an algebraic coordinate grid, which makes the derivation of an equation of a parabola possible. Sasha and Keoni then derive the equation of:
o particular parabolas in Lessons 3 and 4;
o any parabola with vertex (0,0) in Lesson 5; and
o any parabola with vertex (h,k) in Lesson 9.
• CCSS.M.HSG.GPE.B.4. Use coordinates to prove simple geometric theorems algebraically.
Sasha and Keoni use the coordinates on an algebraic Cartesian grid, along with the definition of a parabola, to validate that three points are on the parabola.
• CCSS.M.8.G.B.8. Apply the Pythagorean Theorem to find the distance between two points in a coordinate system. Keoni and Sasha use the Pythagorean theorem, along with the coordinate system and the definition of a parabola, to determine the x-value for a point on the parabola given its y-value.
Common Core Math Practices
CCSS.Math.Practice.MP5 Use appropriate tools strategically.
In this lesson, Sasha and Keoni use an important mathematical tool— the Pythagorean theorem. A discussion in the lesson models an important habit of mind related to tool use. Specifically, when Keoni and Sasha are stumped about how to measure the length of a diagonal line segment from a point on the parabola to the focus [1:46, Episode 2], their teacher encourages them to write down everything they know [1:56, Episode 2] and articulate what they are trying to find [2:53, Episode 2]. In the process, a right triangle emerges on the grid, with two sides of known length and one of unknown length. This practice of analyzing the situation prepares Sasha and Keoni to strategically apply the Pythagorean theorem once the teacher suggests its use [3:56, Episode 2].
