Graphing from the Factored Form

5 min

Narrative

This Warm-up refreshes the work in an earlier lesson. It prepares students to deepen their understanding of the factored form and the intercepts of a graph that represents a quadratic function.

As students work, notice how they find the yy-coordinate of the yy-intercept. Identify students who do so by evaluating w(0)w(0). Ask them to share during a whole-class discussion.

Launch

Arrange students in groups of 2. Give students quiet work time and then time to share their work with a partner. Follow with a whole-class discussion.

Student Task

Here is a graph of a function, ww, defined by w(x)=(x+1.6)(x2)w(x)=(x+1.6)(x-2). Three points on the graph are labeled.

<p>Coordinate plane, x, y. A graph of a quadratic function, opens up. Points on the x axis labeled a, comma b, and c comma d. A point on the y axis labeled e comma f.</p>

Find the values of a,b,c,d,ea,b,c,d,e, and ff. Be prepared to explain your reasoning.

Sample Response

a=-1.6a=\text-1.6, b=0b=0, c=2c = 2, d=0d=0, e=0e=0, f=-3.2f=\text-3.2

Activity Synthesis (Teacher Notes)

Invite students to share their responses and reasoning. If not mentioned in students’ explanations, emphasize that:

  • bb and dd must be 0 because the yy-coordinate of the xx-intercepts of the graph of any function is 0.
  • ee must be 0 because the xx-coordinate of the yy-intercept of the graph of any function is 0.
  • aa and cc correspond to the 1.6 and 2 in the factored expression. Students may reason that the graph shows that the positive xx-intercept is farther away from 0 than the negative xx-intercept, suggesting that cc is 2 and aa is -1.6. They may also use their observations from an earlier lesson: that when a factor in the expression shows a negative sign or subtraction, a corresponding intercept takes a positive value, and when a factor shows a positive sign or addition, a corresponding intercept takes a negative value. Either explanation is reasonable at this point.
  • (0,f)(0,f) is the yy-intercept, so the value of ff can be found by evaluating w(0)w(0), which is -3.2, because (0+1.6)(02)=(1.6)(-2)=-3.2(0+1.6)(0-2) = (1.6)(\text-2) = \text-3.2.
Anticipated Misconceptions

Some students may think that the numerical values in the equation correspond directly to the xx-intercepts in the graph and incorrectly state that a=-2a= \text-2 and c=1.6c=1.6. Remind them that a graph shows all pairs of xx and yy values that make the equation true. Consider asking these students to try substituting -2 for xx and evaluating the expression to verify that w(-2)=0w(\text-2)=0.

Students will have opportunities to attend to the signs or the operations in quadratic expressions in factored form, so it is not essential that this misconception is corrected at this moment.

Standards
Addressing
  • A-SSE.A·Interpret the structure of expressions
  • HSA-SSE.A·Interpret the structure of expressions.

15 min

15 min