Lesson Plan:

The Case of the Missing Rectangle Side

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August 15, 2015
by Susan Russell
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August 15, 2015
by Susan Russell

Learning Objectives

Students will be able to find the missing side of a rectangle given the rectangle’s area and measure of one side by applying the area formula.


Introduction (10 minutes)

  • Prior to the lesson, draw a large rectangle to display.
  • Tell students that today they will be on the case of the missing rectangle side.
  • Inform them they will need their multiplication, division, and reasoning skills to solve the case.
  • Display the large rectangle. Add to the fun by drawing a magnifying glass, and tell students that they are officially on the case of the “missing rectangle side."
  • Tell students that the only clues they have on the case is the area of the rectangle and the measure of one of the rectangle’s sides.
  • Write down the label area underneath the rectangle, defining it as the space inside the shape.
  • Next, review the formula for area.
  • Write down the area formula. Demonstrate an example of solving for area.

Explicit Instruction/Teacher Modeling (10 minutes)

  • Begin by writing down several area problems.
  • Ask students to solve the problems.
  • On your chart paper, draw a rectangle. List one side and display the area.
  • Next to the unknown rectangle side, write an X.
  • With student input, develop a formula for the missing side.
  • Display your formula on your chart paper. For example, 12 x (X) = 36.
  • Ask students how they think this formula can be solved.
  • Discuss strategies, and solve.

Guided Practice/Interactive Modeling (10 minutes)

  • Draw another rectangle with an unknown side.
  • Display one side and the area of the rectangle.
  • Ask students to solve the problem on their own.
  • Check their answers.

Independent Working Time (15 minutes)

  • Ask students to draw 4 rectangles on a piece of notebook paper.
  • Direct your students to write down the area and the measure of one side next to the rectangle and an X for the missing side.
  • Have students exchange papers with partners.
  • Instruct your students to solve for all missing sides on their papers.
  • After they have completed their work, have them return to their partners to correct the work.
  • Have your students discuss any errors with each other.
  • Walk around, and monitor student discussion. Intervene with struggling students as necessary.
  • Collect student work to assess.



  • Enrichment: Ask a small group to develop a real life story problem in which there is an unknown variable for a rectangle where the area and one side is given. In the whole group, present the story problem the small group has written, and have all students solve.
  • Support: Students may struggle solving for the unknown side if they do not have strong basic multiplication and division skills. Work with them in a small group to review these skills.


Assessment (10 minutes)

  • Look over student work.
  • Provide feedback on student papers.

Review and Closing (10 minutes)

  • Take the story problems that your advanced students wrote during enrichment.
  • Have students come up and read their story problem to the class.
  • Solve the problems as a whole group.

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