1. 11-5 Area of Triangles and Trapezoids
Pages
Indicator(s)-
M6 Use strategies to develop formulas for finding area of trapezoids
P8 Use formulas in problem solving

2. Area of Triangles A=½bh
A triangle is a three-sided polygon.
The area of a triangle can be defined by half of the product of the length of its base and its height. The height is always perpendicular to the base, exactly like the base and height of a
parallelogram.
A=½bh

3. Example
Find the area of the triangle below. Round to the nearest tenth if necessary.
A=½bh
A=½(9)(3.2)
A=½(28.8)
A= 14.4 cm2
The area of the given triangle is 14.4 cm2
3.2 cm
9 cm.

4. Area of Trapezoids A=½h(b1+b2)
A trapezoid has two (2) bases, b1& b2. The height of the trapezoid is the perpendicular distance between the two bases.
A=½h(b1+b2) b1 h b2

5. Pettit 11-5 Notes
Example
Find the area of the trapezoid below. Round to the nearest tenth if necessary.
A=½h(b1+b2)
A=½(3)( )
A=½(3)(11.6)
A= 17.4 m2
The area of the given trapezoid is 17.4 cm2
4 m
3 m
7.6 m

6. Example
The shape of the state of Montana resembles a trapezoid. Estimate its area in square miles.
A=½h(b1+b2)
A=½(285)( )
A=½(285)(1021)
A≈ 145,493 mi2
The area of the Montana is about 145,493 mi2.
542 mi
285 mi
479 mi

7. What if… A=½bh A=½(13 in)(1 ft) CONVERT & SOLVE! A=½(13 in)(12 in)
you are dealing with two different measurements?
Find the area of the given triangle. Round to the nearest tenth if necessary.
A=½bh
A=½(13 in)(1 ft)
CONVERT & SOLVE!
A=½(13 in)(12 in)
A= 78 in2
1 ft
13 in

8. Challenge…
Find the area of the trapezoid shown. Round to the nearest tenth if necessary.
A=½h(b1+b2)
HINT: To find the height, you will have to use the Pythagorean Theorem.
9cm
10 cm
6 cm