Lesson 21: Understanding Area
Understanding Area
Lesson 21Topics
x The area congruence and area addition postulates.
x The areas of rectangles, triangles, polygons and regular polygons.
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x area of a rectangle: length × width.
x area of a triangle:^12 EDVHîKHLJKWͼ7KLVIRUPXODDSSOLHVQRPDWWHUZKLFKVLGHLVFRQVLGHUHGWKHEDVHͽ
x DUHDRIDSRO\JRQ 6XEGLYLGHLQWRWULDQJOHV
x area of a regular NJRQ 6XEGLYLGHN triangles, each with its apex at the center of the polygon.
Summary
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rely on identifying key beliefs we hold about what area means and how we operate with it. In this lesson, we
establish two fundamental area postulates, make a single declaration about the areas of rectangles, and explore
the concept of area for polygons.
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A parallelogram has height h and “base” of length b, as shown in
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Draw a diagonal, as shown in )LJXUHWRGLYLGHWKH¿JXUH
into two triangles.
Each triangle has base b and height h and, thus, area^12 bh.
By the area addition postulate, the area of the parallelogram is(^1122) bh bh bh.
b
h
b Figure 21.1
b
h
b Figure 21.2