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

Surface Area Calculation with Progressive Ratios

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.

aaabbbccc

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
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 written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

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.

aaabbbccc

2

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 .

3

Final Answer

a=7.06, b=8.47, c=10.17

Key Points to Remember

Essential concepts to master this topic
  • Formula: Surface area = 2(ab + bc + ca) for rectangular prism
  • Technique: Express all sides in terms of a: b = 1.2a, c = 1.44a
  • Check: Substitute values: 2(1.2 + 1.728 + 1.44) × 49.93 = 436 ✓

Common Mistakes

Avoid these frequent errors
  • Calculating percentage increases incorrectly
    Don't add 20% by doing b = a + 0.2 = wrong ratio! This treats percentage as a flat addition instead of multiplication. Always multiply: b = 1.2a means b is 120% of a, not a + 0.2.

Practice Quiz

Test your knowledge with interactive questions

A cuboid is shown below:

222333555

What is the surface area of the cuboid?

FAQ

Everything you need to know about this question

Why does c = 1.44a instead of 1.4a?

+

Because c is 20% longer than b, not 20% longer than a! First find b = 1.2a, then c = 1.2b = 1.2 × 1.2a = 1.44a. The increases are progressive, not independent.

What if I get a negative value under the square root?

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This would mean your setup is wrong! Check that you've correctly combined like terms and that your surface area formula is 2(ab + bc + ca), not just (ab + bc + ca).

How do I know which variable to solve for first?

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Always express everything in terms of the simplest variable - usually the one without percentage increases. Here, express b and c in terms of a, then solve for a first.

Can I work backwards from the answer choices?

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Yes! This is a great checking method. Pick an answer choice for a, calculate b and c, then check if 2(ab+bc+ca)=436 2(ab + bc + ca) = 436 . If it works, you found the right answer!

Why do we multiply by 2 in the surface area formula?

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A rectangular prism has 6 faces: 2 faces of area ab, 2 faces of area bc, and 2 faces of area ca. So total area = 2ab + 2bc + 2ca = 2(ab + bc + ca).

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