- 1). Measure the sides of one triangle using your ruler. Write down the measurement of each side. For example, the measurements of the sides of the first triangle might be 4 cm, 5 cm and 7 cm.
- 2). Add the measurements of the sides of the first triangle together. In the example, 4 + 5 + 7 = 16.
- 3). Divide the sum of the three sides of the first triangle by 2. In the example, 16/2 = 8. This number is the semiperimeter of the first triangle.
- 4). Subtract the measurement for one side of the first triangle from the semiperimeter of the first triangle. In the example, 8 -- 4 = 4.
- 5). Subtract the measurement for a second side of the first triangle from the semiperimeter of the first triangle. In the example, 8 -- 5 = 3.
- 6). Subtract the measurement for the third side of the first triangle from the semiperimeter of the first triangle. In the example, 8 -- 7 = 1.
- 7). Multiply the three numbers resulting from subtracting each side from the semiperimeter by each other and times the semiperimeter. In the example, 4 x 3 x 1 x 8 = 96.
- 8). Take the square root of the multiplied value of the four numbers using a calculator. In the example, the square root of 96 is 9.79. The area of the first triangle is 9.79 square centimeters.
- 9). Measure the sides of the second triangle using your ruler. Write down the measurement of each side. For example, the measurements of the sides of the second triangle might be 3 cm, 4 cm and 5 cm.
- 10
Add the measurements of the sides of the second triangle together. In the example, 3 + 4 + 5 = 12. - 11
Divide the sum of the three sides of the second triangle by 2. In the example, 12/2 = 6. This number is the semiperimeter of the second triangle. - 12
Subtract the measurement for one side of the second triangle from the semiperimeter of the second triangle. In the example, 6 -- 3 = 3. - 13
Subtract the measurement for the second side of the second triangle from the semiperimeter of the second triangle. In the example, 6 -- 4 = 2. - 14
Subtract the measurement for the third side of the second triangle from the semiperimeter of the second triangle. In the example, 6 -- 5 = 1. - 15
Multiply the three numbers resulting from subtracting each side from the semiperimeter by each other and times the semiperimeter. In the example, 3 x 2 x 1 x 6 = 36. - 16
Take the square root of the multiplied value of the four numbers using a calculator. In the example, the square root of 36 is 6. The area of the second triangle is 6 square centimeters. - 17
Add the area of the first triangle to the area of the second triangle. In the example, 9.79 + 6 = 15.79. The area of the two triangles is 15.79 square cm.
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