Lesson Description
Keoni and Sasha determine a general method for representing the equation for a parabola with a vertex on the origin and a focus p units above the vertex.
Targeted Understandings
This lesson can help students:
- Interpret the meaning and use of an equation that they derive to represent a family of parabolas with vertex at the origin and focus p units above the vertex (y = x2/(4p)).
- Conceive of the parameter p in the equation y = x2/(4p) as a distance between the vertex and focus of a parabola. The value of p can vary, but once it is set (e.g., when p = 2), then an equation is defined (e.g., y = x2/8) that represents a unique parabola.
- Relate geometric features of parabolas to elements of corresponding equations. For example, the equation y = x2/8 can also be expressed as y = x2/(2•4), where the 2 represents the number of units between the focus and vertex of the parabola.
Common Core Math Standards
• CCSS.M.HSG.GPE.A.2: Derive the equation of a parabola given a focus and directrix.
In Episode 6, Sasha and Keoni derive the equation (in two forms) for the family of parabolas with vertex (0,0) and focus p units above the vertex, namely y = x2/(4p) and x = √(4py). They do this by generalizing the method they used to develop an equation for a parabola with p = 1 in Lessons 3 and 4 (y = x2/4 and x = √(4y)), for a parabola with p = 2 in Episode 2 of this lesson (y = x2/8 and x = √(8y)), and for a parabola with p = 3 in Episode 4 (y = x2/12 and x = √(12y)).
• CCSS.M.HSA.APR.A.1: Perform arithmetic operations on polynomials.
Sasha and Keoni multiply binomials, such as (y + 2)2 and (y + p)2, in service of deriving an equation for a particular parabola (in Episodes 2 and 4) or a family of parabolas (Episode 6).
Common Core Math Practices
CCSS.Math.Practice.MP7. Look for and make use of structure.
In this lesson, Sasha and Keoni make use of mathematical structure on two levels. First, they discern a pattern relating the equations of three parabolas (y = x2/4, y = x2/8, and y = x2/12) with the corresponding distance between the vertex and focus (respectively, p = 1; p = 2; and p = 3). Specifically, they see the structure of 1•4, 2•4 and 3•4 in the denominators of their equations, which is the p-value of the parabola multiplied by 4. Consequently Keoni and Sasha conjecture that the general equation of a parabola with vertex (0,0) and focus p units above the vertex will be y = x2/(4p). Then they look for and make use of structure on a second level. To derive the equation y = x2/(4p), they need to identify the lengths of the sides of a right triangle with hypotenuse connecting a general point on the parabola with the focus. To accomplish this challenging task, Sasha and Keoni see a pattern in similar quantities that they identified for particular parabolas, which they generalize to define lengths involving the parameter p, such as y – p and y + p.
