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Use the figure at the right.

We will find the areas of triangles

Review Vocabulary: Triangle

In lesson, we learned that the area of a triangle is related to the area of triangle and parallelogram. To find the area of a triangle, multiply the measures of the base and the height and divide by two.

The base of a triangle can be any one of its sides. The height is the shortest distance from a base to opposite vertex.

 

 

Key Concept: Area of a Triangle.

Words: The area of a triangle A is one-half the product of the base b and its height h.

Symbols:

FLAGS What is the area of the triangle on the flag?

Area of a triangle

Replace b with 6, h with 4.

Multiply 6 by 4.

Divide by 2.

.

The area of the triangle on the flag is 12 square inches (äþéì-2,54 ñì).

 

Area of a triangle

Replace b with 34, h with 12.

Multiply 34 by 12.

Divide by 2.

.

The area of the triangle is 204 square feet (ôóò-30,48 ñì).

 

7. The side of a triangle boat sail measures 9 feet and the height measures 6 feet. How much fabric was used to make the sail?

Home task

8. Talk Math. Explain how two different triangles can have an area of 24 square feet.

(If two triangles have such side, will be similar)

Use the figure at the right.

17. Find the area of the largest triangle.

18. Find the area of the green triangle.

19. What is the area of one small triangle?

 

20. What are the areas of the triangles and the rectangle on Desiree’s sketch?

21. CHALLENGE (Problem) The area of a triangle is 16 square feet, and its height is 4 feet. What is the length of the base?

22. WHICH ONE DOESN’T BELONG? Which figure does not belong with the other two? Explain your reasoning (discuss).

Practice:

23. Write Math Write about a real-world problem that could be solved by finding the area of a triangle.

 

24 Find the area of the triangle.

24 Find the area of the triangle.

m?

Example 1: Find the area of an acute triangle with a base of 15 inches and a height of 4 inches.
Solution:
= · (15 in) · (4 in)
= · (60 in2)
= 30 in2

 

Example 2: Find the area of a right triangle with a base of 6 centimeters and a height of 9 centimeters.
Solution:
= · (6 cm) · (9 cm)
= · (54 cm2)
= 27 cm2

 

Example 3: Find the area of an obtuse triangle with a base of 5 inches and a height of 8 inches.
Solution:
= · (5 in) · (8 in)
= · (40 in2)
= 20 in2  

 



 

26. What is the length of the base of a triangle with height 20 cm and area 150 square cm?

27. What is the length of the base of a triangle with height 18 m and area 99 square m?

28. What is the area of a triangle with base 29.9 cm and height 25.6 cm?

29. What is the height of a triangle with base 24 mm and area 132 square mm?

30. What is the length of the base of a triangle with height 16 mm and area 136 square mm?

31. What is the height of a triangle with base 12 mm and area 150 square mm?

32. What is the height of a triangle with base 26 mm and area 377 square mm?

33. What is the area of a triangle with base 20.3 mm and height 5.6 mm?

34. What is the area of a triangle with base 11 m and height 23


Date: 2015-12-24; view: 656


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