Section 10.2

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Section 10.2 Area Triangles and Trapezoids

• pages 527 - 531

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In section 9.3, we learned how to name a triangle
by its sides and angles.
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The Bermuda Triangle is a triangular region
between Bermuda, Florida, and Puerto Rico.
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Last week we learned how to find the area of
parallelograms.  Today we are going to discuss
how to find the areas of triangles and
trapezoids.  When looking at a parallelogram, we
notice that the diagonal of a parallelogram
divides the parallelogram into two congruent
triangles.   Thus, the area of each triangle is
half the area of the parallelogram.
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The area of a triangle equals half the product of
any base length b and the corresponding height h.
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Example 1 Find the area of the triangle.
The area is 12 cm2.
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.                                    
Check Understanding a. Find the area of the
triangle.
b. Find the area of the triangle.
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Example 2
How much siding does a builder need to cover the
side of the house shown at the left?
Add to find the total 72 160 232.
The builder needs 232 ft2 of siding.
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A parallelogram can be divided into two congruent
trapezoids.  The area of each trapezoid is
one-half the area of the parallelogram.
The two parallel sides of a trapezoid are its
bases.  If we call the longer side b1 and the
shorter side b2, then the base of the
parallelogram is b1b2.
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Area of a Trapezoid
The area of a trapezoid is half the product of
the height and the sum of the lengths of the
bases. A ½ h(b1 b2)
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Example 3 Finding area of a trapezoid
The Erie Canal opened in 1825 and was hailed as
an engineering marvel. Below is a cross section
of the Erie Canal. Find the area of the
trapezoidal cross section.
The area of the cross section is 136 ft2.
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.                                 
.                                     
Check Understanding
a. Find the area of the trapezoid.            
b. Find the area of the trapezoid.
                    
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Homework

• pages 530 - 531
• s 1-10, 20-28
• DRAW DIAGRAMS and
• write problems and answers, EXCEPT for word
  problems and multiple-choice problems
• SHOW ALL WORK!
• failure to write problems and/or show work will
  result in either receiving partial or no credit