Introduce the definition of area as the space within a flat or two-dimensional object or shape. Ask the children to point out two-dimensional objects within the classroom.
Discuss how you might use the area of a space or shape in everyday life. For example, explain how they would use the formula to determine the amount of carpet or tile needed to cover a floor or how much wallpaper would be needed for a wall.
Familiarize the children with the concept of area by finding the area of a rectangle. Draw a rectangle and display it on the overhead projector. Label the sides as "Base" and "Height," accordingly. Include only whole squares within the rectangle.
Count the squares aloud with the class to find the area inside the rectangle. Write the formula for the area of a rectangle as "base x height = area." Instruct the children to use the formula to find the area.
Call attention to the fact that the answer acquired by counting the number of squares is the same answer achieved using the formula.
Draw a straight line from one corner of the rectangle to the opposite corner to divide the rectangle in half, creating two congruent triangles.
Write the formula "(base x height) /2 = area" and instruct the children to copy it on the graph paper in front of them. Discuss the relationship between the area of a rectangle and the area of a triangle.
Introduce the parts of a triangle used to determine the area of a triangle, including the base and height.
Define or review other related terms such as opposite, adjacent, side and angle, with an emphasis on right or perpendicular angles.
Explain that the height of a triangle is at a right angle to its base. Draw examples of right triangles as well as obtuse and acute triangles.
Show the class how to use a protractor to distinguish the difference between the different types of angles in each example.
Instruct the children to draw a triangle on their graph paper. Specify that the base should begin and end at the line between squares on the graph. Label the base with the letter “B.”
Tell the class to place the protractor over the triangle so that the base lines up with the bottom edge of the protractor.
Ask the children to decide if either of the angles adjacent to the base are right angles. If it is a right angle, mark the adjacent side with an “H” for height.
Teach the children to use their protractors to create a line that will be used as the height when the triangle does not contain a right angle. Label the line with an "H." Help the children identify the height of their own triangles.
Instruct the class to rewrite the formula for the area of a triangle, replacing the letters with the appropriate values for "B" and "H." Solve the equation to find the area and check their answers.
Encourage the children to draw different types of triangles on their graph paper and find the area for each one. Practice until the children are comfortable with the concept.