Area of Triangles

andTrapezoids

Area of a Triangle

Formula to MEMORIZE!!!!

hbA 2

1

What do the letters stand for?

•A is area•b is base of the triangle•h is height of the triangle

Obtuse Triangle

b

h

Right Triangle

h

b

Acute Triangle

b

hb h

Question 1

• What do you notice about the b and the h in the different triangles?

* b and h will ALWAYS form a right angle*

Question 2

• Is the height, h, a part of the triangle?

- Only for right triangles. Otherwise, h is an imaginary line.

* height, h, is also referred to as the altitude of a triangle.

Example 1

• What is the area?

16

12

96)1216(21 Answer:

Example 2

• Find the area.

9 cm

8 cm

Answer: 236)89(21

cm

Perimeter of Triangles

• To find the perimeter, ADD the 3 SIDES of the triangle.

* Be careful, sometimes h is not one of the sides of the triangle.*

Example 1

Find the perimeter.

8 in

4 in6 in

19 in

Answer: in331986

Area of a Trapezoid

Formula to MEMORIZE!!!!

)(21

21 bbhA

What is b1 and b2?

• b1 and b2 represent the parallel sides of a trapezoid.

b1

b2

h

b2

b1

Question 1

• What do you notice about b1 and b2 and the h in the trapezoid?

* they form a right angle*

Question 2 & 3• What happens if I put 2

trapezoids together?- They form a rectangle.• What can you conclude about the

area of 1 trapezoid versus a rectangle?

- The area of a trapezoid would be HALF the area of a rectangle.

Question 4

• Why do you ADD b1 and b2 in the formula?

- Adding b1 and b2 represents the base of the rectangle. That’s why the formula is similar to the area of a rectangle formula.

Directions

• Find the perimeter and area of the following examples.

Example 1

6 m

12 m

8 m9 m

Perimeter:

Area:

35 m272m

Example 2

Perimeter:

Area:

26 m25.32 m

4 m

9 m

6 m 7 m

5 m