6th Grade Math Textbook, Fundamentals

Lesson 10-7 for exercise sets. &KDSWHU 

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The area of the trapezoid at the right is 184 in^2. Find the missing base length.

A (b 1 b 2 )h Use the formula for area of a trapezoid.

184  (x17)16 Substitute the known values.

184  • 16(x17) Apply the Commutative Property.

184 8(x17) Simplify.

184  8 x(8 • 17) Apply the Distributive Property.

184  8 x 136 Simplify.

184  136  8 x 136  136 Subtract 136 from both sides.

48  8 x Simplify.

 Divide both sides by 8.

6 x Simplify.

The missing base length is 6 inches.

1

2

8 x

8

48

8

1

2

1

2

1

Think

1 cm • 1 cm 1 cm^2

10 mm • 10 mm 100 mm^2

So the number of square centimeters

is the number of square millimeters

divided by 100.

b 2  17 in.

b 1  x in.

h  16 in.

Find the area of the polygon.

1.Trapezoid: b 1 30.3; b 2 41.7; h4.

2.Triangle: b15.6; h24.

3.Trapezoid: b 1 52; b 2 63; h10.

Find the unknown dimension of the polygon.

4.Triangle: b60.25 mm; h?; A964 mm^2.

5.Triangle: b?; h101 in.; A1116.5 in.^2

6.Trapezoid: b 1 225 ft; b 2 375 ft; h?; A3300 ft^2.

7.Discuss and Write A trapezoid has bases that measure 16 cm and 20 cm;

it has a height of 4 cm. If you were to double the length of each base, how

would the area of the trapezoid be affected? What if you were to double

only the height? What if you were to double both the bases and the height?

1

2

Remember that you can rename the area of a figure by

finding its equivalent in different units. Since 1 cm 10 mm,

then 1 cm^2 100 mm^2. So to find the area of the trapezoid

on the preceding page in square centimeters, divide:

73.5  100 0.735

The area of the trapezoid is 0.735 cm^2.

As with parallelograms, if you know the area of a triangle or

of a trapezoid and either the base or the height is unknown,

you can use an area formula to find the unknown dimension.