Area of a Triangle – Gr. 7 Math, White Oaks
Subject: Math
Strand(s): Measurement
Grade: 7
Student Teacher: Kevin Watt
Associate Teacher: Brenda Hinschberger
Expectations:
 solve problems related to the calculation and comparison of the perimeter and the
area of two-dimensional shapes 1

develop the formulas for finding the area of a parallelogram and the area of a
triangle 1
 develop and apply the formula for finding the area of a triangle 2
Assessment Tasks / Criteria:
Students complete worksheet 14.1. They add at least three triangles of equal area
and justify their additions in at least two different ways. Students should be reminded
that they could use the following techniques:



counting squares for area
referencing base and height
using partner quadrilaterals
1 mark is given for each triangle drawn. Students get 1 extra mark each per
legible, explanatory justification. 1 extra mark can be achieved for excellent notation,
including the labeling of base and height, or above-average explanations.
Teaching / Learning Strategies:
The following words will be added to the concept chart:
Obtuse Triangle: A triangle in which exactly one angle is greater than 90 degrees
Acute Triangle: A triangle in which all of the angles are less than 90 degrees.
Equilateral Triangle: An equilateral triangle is one that has all 3 sides the same length: All the angles
of the triangle are the same too. They are all 60 degrees.
On the board, the teacher will draw a number of different triangles:
1
2
Ministry of Education and Training. The Ontario Curriculum, Grades 1-8, Mathematics. Ontario: 1997.
TIPS: Section 3 – Grade 7. Day 14: Venn Diagrams.
Characteristics are discussed with the following questions:
Question: What are the distinguishing features of each kind of triangle?
Answer:



Obtuse: the top tip of the triangle falls on one side of the base
Acute: the top tip of the triangle falls inside the base, but not right
in the middle.
Right: the top tip of the triangle fills exactly in the middle of the
base.
REVIEW:
Students estimate the area of objects in the class (examples: desktop,
binder cover, door, eraser, ceiling, PMI chart). Students classify the items into
three different groups, with the teacher acting as a scribe on the chalkboard:


MM2
Eraser
Flattened-out
straw


CM2
Desktop
Binder Cover


M2
Door
Ceiling
THINK/PAIR/SHARE:
On grid paper, students will draw:




Square
Rectangle
Rhombus
2 parallelograms
Students cut out the shape and split them into two equal triangles. The teacher
models this process for a parallelogram and a rhombus using firm paper such as
Bristol board. The pairs predict the area of each triangle that they have created.
CLASS DISCUSSION:
The class discusses a general rule for the area of a triangle using their own words.
For example, the general rule may be that the triangle’s area is always ½ of the
partner quadrilateral. The teacher draws triangles and invites volunteers to use
dotted lines to complete a partner quadrilateral, noting the base and height. The
teacher encourages various solutions and predicts and calculates areas.
ASSESSMENT:
Students complete worksheet 14.1 Students add triangles of equal area
and justify their additions in at least two different ways. Students should be
reminded that they can use the following techniques:



counting squares for area
referencing base and height
using partner quadrilaterals
HOMEWORK:
For homework, students will:


draw triangles that are 2x the area of a parallelogram
draw triangles that are equal to the area of a parallelogram
In each case, the results can be explained in text or by the use of a fully notated
diagram.
Accommodations / Modifications:
None.
Theme / Topic / Resources:
The topic for this lesson is “perimeter and area of triangles”.
Resources include:



Photocopies of 14.1: Triangles.
Graph paper.
Firm paper (ex: Bristol board)