Area of Composite Shapes
Lesson 4 of 19
Objective: SWBAT: • Find the area of a parallelogram by using the formula. • Find the area of a trapezoid and composite shapes by decomposing them into triangles, rectangles, squares, and/or parallelograms.
See my Do Now in my Strategy folder that explains my beginning of class routines.
Often, I create do nows that have problems that connect to the task that students will be working on that day. Today I want students to solve problems where they have to determine when to calculate perimeter and when to calculate area. When finding perimeter, a common mistake is that students include the height inside the triangle so that they add four measurements together. For the area, a common mistake is for students to just multiply the base times the height. Other students may struggle to identify the base and the height. I look for these mistakes and may present one as my answer if I see multiple students making one of these mistakes while I circulate during the do now.
I call on a student to share one idea. That student then calls on the next student to share his/her idea. I encourage students to build on what their classmates have said by using sentence starters like, “I agree/disagree with __________ because…” and “My idea connects with ____________’s idea…” Students are engaging in MP3: Construct viable arguments and critique the reasoning of others.
- Before this lesson make copies of the parallelograms and trapezoids.
- I give each student a copy of these shapes and a pair of scissors.
- I remove the words in italics on students’ notes before I print them.
I want students to take time to figure out a strategy to find the area of the parallelograms and trapezoids. Some students will split a parallelogram into two triangles and a rectangle to find the area. Other students may count the whole squares and ½ squares. Other students may recognize they can move one of the triangles to the other side of the shape to create a rectangle to find the area. Students are engaging in MP8: Look for and make sense of repeated reasoning.
For the trapezoids some students may create two triangles and a rectangle. Some struggling students may try to estimate the area by counting whole squares and partial squares. Other students may recognize that with trapezoid III they can move a triangle to create a rectangle, although this strategy does not work with the other trapezoids.
I choose students to share their strategies under the document camera. I look for a student who created rectangles and triangles. I also look for a student who moved a triangle to create a rectangle. If no one uses this strategy I cut out the parallelogram and show them.
When I define parallelogram I also introduce the formula for area of base times height. When I introduce the trapezoid I do not introduce a formula. I find that many students get confused with this formula. Instead, I encourage students to break trapezoids up into triangles and a rectangle/square.
Students work on breaking the composite shapes into shapes they can use to find area. Most students will break this shape into 2 right triangles and a rectangle. Some students may create triangles that are not right triangles and a rectangle. They will struggle to calculate the area because the grid will not allow them to accurately calculate the area.
Area of Composite Shapes
We quickly review the strategy of breaking composite shapes that we know. There is more than one way to do this with many composite shapes. I emphasize to students that they need to find a way that is efficient and works with the measurements that are given. Sometimes they will have to do detective work to figure out missing measurements.
Some students look at the composite shape and immediately say, “I don’t know how to find the area of that kind of shape.” I have two students come and show their different strategies for breaking up the shape from the previous section.
- Before this lesson I Post A Key.
I have a student read over the directions. I review expectations and students start working independently. Students are engaging in MP6: Attend to precision and MP7: Look for and make use of structure.
As students work I walk around to monitor student progress and behavior. If students are struggling, I may ask them one or more of the following questions:
- How can you break this shape up into shapes you know how to deal with?
- What are the base and height of the triangle? How do you know?
- How can you find the area of this shape? Why does that work?
When students complete their work, they raise their hands. I quickly scan their work. If they are on track, I send them to check with the key. If there are problems, I tell students what they need to revise. If students successfully complete the chart they can work on the challenge questions.
Closure and Ticket to Go
I ask students to turn to problem 7. I ask students to share their thinking about finding the area of this shape. Students participate in a Think Pair Share. I want students to recognize that they can break this shape into a parallelogram, a rectangle, and a triangle. Some students may struggle to identify the base of the triangle. By comparing the two parallel lines, we can see that the base of the triangle is 7 yards. Students are engaging in MP3: Construct viable arguments and critique the reasoning of others and MP7: Look for and make use of structure.
I pass out the Ticket to Go and the Homework.