Area and Perimeter of Triangles: Triangles Inside and Out

This is one lesson in a larger Unit Plan that covers many properties of triangles and several other theorems.  It has proofs, real world problems, constructions, a unit project, assessments, and more.  In Lesson 7, “Triangles Inside and Out (Area and Perimeter of Triangles)”, students investigate the relationship between triangle perimeter and area, and includes real world connections.  The entire unit plan can be found hereThis lesson includes the use of an applet that allows for manipulation of a triangle on a computer. This could be used individually in the computer lab, or in a group setting on an interactive board.

Standards & Objectives

Academic standards
Fluently divide multi-digit numbers using the standard algorithm.
Fluently add, subtract, multiply, and divide multi-digit decimals using the standard algorithm for each operation.
CLE 3108.1.6
Employ reading and writing to recognize the major themes of mathematical processes, the historical development of mathematics, and the connections between...
CLE 3108.4.1
Develop the structures of geometry, such as lines, angles, planes, and planar figures, and explore their properties and relationships.
CLE 3108.4.6
Generate formulas for perimeter, area, and volume, including their use, dimensional analysis, and applications.
GLE 0506.2.3
Develop fluency with division of whole numbers. Understand the relationship of divisor, dividend, and quotient in terms of multiplication and division.
SPI 0606.2.3
Solve problems involving the addition, subtraction, multiplication, and division of decimals.
SPI 0606.2.4
Solve multi-step arithmetic problems using fractions, mixed numbers, and decimals.
Fluently divide multi-digit numbers using a standard algorithm.
Fluently add, subtract, multiply, and divide multi-digit decimals using a standard algorithm for each operation.
Alignment of this item to academic standards is based on recommendations from content creators, resource curators, and visitors to this website. It is the responsibility of each educator to verify that the materials are appropriate for your content area, aligned to current academic standards, and will be beneficial to your specific students.
Essential and guiding questions: 
  • How do area and perimeter compare and contrast?
  • How are area and perimeter related to other geometric topics?

Lesson Variations

Blooms taxonomy level: 

Helpful Hints


  • Worksheets
  • calculator
  • computer with internet access
  • dot paper
  • geoboards