Developing Formulas for

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Developing Formulas for Triangles and
Quadrilaterals
9-1
Warm Up
Lesson Presentation
Lesson Quiz
Holt Geometry
2
Warm Up Find the unknown side length in each
right triangle with legs a and b and hypotenuse
c. 1. a 20, b 21 2. b 21, c 35 3. a
20, c 52
c 29
a 28
b 48
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Objectives
Develop and apply the formulas for the areas of
triangles and special quadrilaterals. Solve
problems involving perimeters and areas of
triangles and special quadrilaterals.
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• Steps (IMPORTANT Do all of these steps on
  homework and the test for full credit!!)
• Write out formula
• Substitute in known values
• Solve for desired value

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Area of the rectangle is A b(h)
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Check out http//www.keymath.com/x3338.xml to
learn about the area formula of a parallelogram.
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To find the area formula for a trapezoid,
visit http//www.geogebra.org/en/upload/files/eng
lish/Barbara_Perez/Area_Trapezoid.html
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Example 2A Finding Measurements of Triangles and
Trapezoids
Find the area of a trapezoid in which b1 8 in.,
b2 5 in., and h 6.2 in.
Area of a trapezoid
Substitute 8 for b1, 5 for b2, and 6.2 for h.
Simplify.
A 40.3 in2
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Example 2C Finding Measurements of Triangles and
Trapezoids
Find b2 of the trapezoid, in which A 231 mm2.
Area of a trapezoid
42 23 b2
Subtract 23 from both sides.
19 b2
b2 19 mm
Sym. Prop. of
http//www.geogebra.org/en/upload/files/english/Ba
rbara_Perez/Area_Trapezoid_Practice.html
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http//www.geogebra.org/en/upload/files/english/Ba
rbara_Perez/Area_Kite.html http//www.geogebra.or
g/en/upload/files/english/Barbara_Perez/Area_Rhomb
us.html
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Example 3A Finding Measurements of Rhombuses and
Kites
Find d2 of a kite in which d1 14 in. and A
238 in2.
Area of a kite
Substitute 238 for A and 14 for d1.
Solve for d2.
34 d2
Sym. Prop. of
d2 34
http//www.geogebra.org/en/upload/files/english/Ba
rbara_Perez/Area_Kite_Practice.html