Reflection: Modeling Perimeter of Irregular Rectilinear Shapes - Section 1: Introduction


Instead of showing students how to do the problem first, I let them work on the problem first.  I had to explain to students that the drawings are not to scale - so rely on the labeled dimensions.  Students were trying to measure sides to figure out missing dimensions.  That is a completely reasonable thing to do, but not how these problems work!

Some students found the perimeter by adding 3 + 4 + 6 + 2 thereby overlooking two additional sides.  Some students realized there was a grid they could use.  They made good use of this - in most cases!  

Once students had a chance to work on the problem themselves, the given steps made more sense.  

I'd ask:  

Why might it be helpful to first count the number of sides in the shape?  [So we make sure we include all sides in perimeter.  So we know how many missing side lengths to find]

What were some strategies you used to find the missing sides? [Drawing the shape in a grid.  Make hash marks around the shape.  Joining sides together to find total distance.]

Why do we add up the lengths of all the sides? [to find the perimeter]

  Drawing Are Not to Scale!
  Modeling: Drawing Are Not to Scale!
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Perimeter of Irregular Rectilinear Shapes

Unit 6: Geometry
Lesson 2 of 37

Objective: SWBAT find the perimeter of rectilinear shapes by determining and summing the lengths of all sides

Big Idea: Students use the properties of rectangles to discover missing dimensions.

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7 teachers like this lesson
Math, areas, circumference (Determining Measurements), Geometry, perimeter, Rectilinear, circles, perimeter, Diameter, pi
  40 minutes
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