Big Ideas Math: Modeling Real Life, Grade 8
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7. Perimeters and Areas of Similar Figures
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Exercise 7 Page 86

Practice makes perfect

We are told that two similar triangular regions are prepared for development.

We want to find the cost to surround the forest with a fence. Remember that when two figures are similar, the value of the ratio of their perimeters is equal to the value of the ratio of their corresponding side lengths. P_1/P_2=H/h In this case, P_1 is the perimeter of the grassland, P_2 describes the perimeter of the forest, H represents the height of the grassland, and h indicates the height of the forest. Let's substitute P_1= 240, H= 60 and h= 45 into the equation and solve for P_2.
P_1/P_2=H/h
240/P_2=60/45
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Solve for P_2
240/P_2=60Ă· 15/45Ă· 15
240/P_2=4/3
240/P_2* P_2=4/3* P_2
240=4/3* P_2
240=4* P_2/3
240* 3=4* P_2/3* 3
240* 3=4* P_2
720=4* P_2
720/4=4* P_2/4
720/4=4* P_2/4
720/4=P_2
180=P_2
P_2=180
The perimeter of the forest region is 180 yards. We are told that it costs 6 dollars per foot to install fencing. To find the price to surround the forest with a fence, we need to convert the perimeter from yards to feet by multiplying 180 by 3. 180 yards * 3 feet/1 yard = 540 feet Finally, let's multiply this by 6 to get the cost to surround the forest with a fence. 540 feet * 6 dollars/1 feet = 3240 dollars It would cost $3240 to build a fence around the forest region.
We know that the cost to prepare 1 square yard of grassland is 15 dollars and the cost to prepare 1 square yard of forest is 25 dollars. We want to determine which region is more expensive to prepare. Let's find the ratio of the areas of the similar figures. A_1/A_2=(H/h )^2 In this case, A_1 is the area of the grassland, A_2 describes the area of the forest, H represents the height of the grassland, and h indicates the height of the forest. Let's substitute A_1= 2400, H= 60 and h= 45 into the above equation and solve it for A_2.
A_1/A_2=H/h
2400/A_2=(60/45)^2
â–Ľ
Solve for A_2
2400/A_2=(60Ă· 15/45Ă· 15)^2
2400/A_2=(4/3)^2
2400/A_2=4^2/3^2
2400/A_2=16/9
2400/A_2* A_2=16/9* A_2
2400=16/9* A_2
2400=16* A_2/9
2400* 9=16* A_2/9* 9
2400* 9=16* A_2
21 600=16* A_2
21 600/16=16* A_2/16
21 600/16=16* A_2/16
21 600/16=A_2
1350=A_2
A_2=1350
The area of the forest is 1350 square yards. We can calculate the cost to prepare the forest region by multiplying 1350 by 25. Forest Cost=1350 * $ 25 = $ 33 750 Next, we can find the cost to prepare the grassland region by multiplying 2400 by 15. Grassland Cost= 2400 * $ 15 = $ 36 000 As we can see, it will cost more to prepare the grassland than the forest. $ 36 000 &> $ 33 750 Grassland &> Forest