standard_description: Prove the slope criteria for parallel and perpendicular lines and use them to solve geometric problems (e.g., find the equation of a line parallel or perpendicular to a given line that passes through a given point).
problem_activity: Narrative
The purpose of this lesson is for students to learn stage 1 of the center, How Close? Students pick a given number of digit cards and then choose a subset of those to make an expression that yields a number as close as possible to 20.
Required Materials
Materials to Gather
Connecting cubes or two-color counters
Double 10-frames
Number cards 0–10
Materials to Copy
How Close? Stage 1 Recording Sheet
Launch
Groups of 2
Give each group a set of number cards, two recording sheets, and access to double 10-frames and connecting cubes or two-color counters.
“We are going to learn a game called How Close? Add to 20. Let's play the first round together.”
“First we take out any card that has the number 10. We will not use those cards for the game.”
Display 5 cards.
“I can choose two or three of these cards to add to get as close to 20 as I can. What cards should I choose?”
1 minute: quiet think time
2 minutes: partner discussion
Share responses.
“I write an equation with the numbers I chose and the sum of the numbers.”
Demonstrate writing the equation on the recording sheet.
“The person who gets a sum closer to 20 gets a point for the round. Then you each get more cards so you always have five cards to choose from. Play again. The person who gets more points wins.”
Activity
10 minutes: partner work time
Activity Synthesis
Invite students to share their methods for making a sum close to 20.


aligns: no