Surface Area 436 cm²: Find Sides of Orthohedron with 20% Progressive Increase

Question

Look at the orthohedron shown in the figure below.

Side b is 20% longer than side a.

Side c is 20% longer than side b.

Calculate a, c, and b given that the surface area of the orthohedron is 436 cm².

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Video Solution

Solution Steps

00:00 Find A, B, C
00:04 Expression for size B using A according to the given
00:09 Expression for size C using B
00:14 Substitute the value of B and find the value of C
00:25 This is the expression for size C using A
00:29 Use the formula to calculate the surface area of a box
00:35 Substitute appropriate values and solve to find A
00:46 Simplify what's possible
00:58 Substitute the expressions for B,C using A that we found
01:26 Solve each multiplication separately
01:34 Isolate the unknown A
01:43 This is size A
01:58 Substitute this size in the expression for size B and solve to find B
02:07 This is size B
02:15 Substitute size A in the expression to find size C
02:26 This is size C, and this is the solution to the question

Step-by-Step Solution

First, we'll express b b and c c in terms of a a :
b=1.2aandc=1.44a b = 1.2a \quad \text{and} \quad c = 1.44a

Next, substitute these into the surface area formula for a cuboid:

2(ab+bc+ca)=436 2(ab + bc + ca) = 436

ab=a(1.2a)=1.2a2 ab = a \cdot (1.2a) = 1.2a^2

bc=(1.2a)(1.44a)=1.728a2 bc = (1.2a) \cdot (1.44a) = 1.728a^2

ca=(1.44a)a=1.44a2 ca = (1.44a) \cdot a = 1.44a^2

Substitute these into the formula:

2(1.2a2+1.728a2+1.44a2)=436 2(1.2a^2 + 1.728a^2 + 1.44a^2) = 436

Simplify inside the parentheses:

24.368a2=436 2 \cdot 4.368a^2 = 436

8.736a2=436 8.736a^2 = 436

Solve for a2 a^2 :

a2=4368.736 a^2 = \frac{436}{8.736}

a249.93 a^2 \approx 49.93

Taking the square root, we find:

a49.937.06 a \approx \sqrt{49.93} \approx 7.06

Now, find b b and c c :

b=1.2a=1.2×7.068.47 b = 1.2a = 1.2 \times 7.06 \approx 8.47

c=1.44a=1.44×7.0610.17 c = 1.44a = 1.44 \times 7.06 \approx 10.17

Therefore, the dimensions of the orthohedron are:

a=7.06 a = 7.06 , b=8.47 b = 8.47 , and c=10.17 c = 10.17 .

Answer

a=7.06, b=8.47, c=10.17