Math Problem Statement

Given a cube with a surface area of 96 cm². A and B are midpoints of two adjacent edges. C and D are two further vertices of the cube so that AB is parallel with CD as in the figure below. Find the area of the quadrilateral ABCD.

Solution

To solve this problem, we need to break it down into several steps.

Step 1: Find the Side Length of the Cube

The surface area of a cube with side length ss is given by: 6s2=96 cm26s^2 = 96 \text{ cm}^2 Dividing both sides by 6: s2=16s=4 cms^2 = 16 \Rightarrow s = 4 \text{ cm}

Step 2: Determine the Lengths of ABAB and CDCD

Since AA and BB are midpoints of two adjacent edges, each of these edges has length s=4s = 4 cm, so the length of ABAB is half of the face diagonal of the cube.

The face diagonal ACAC on one face of the cube is: AC=s2=42 cmAC = s\sqrt{2} = 4\sqrt{2} \text{ cm} Since ABAB is half of this face diagonal: AB=422=22 cmAB = \frac{4\sqrt{2}}{2} = 2\sqrt{2} \text{ cm} Similarly, CDCD is also 222\sqrt{2} cm since it is parallel to ABAB and located on a parallel face of the cube.

Step 3: Calculate the Area of Quadrilateral ABCDABCD

The quadrilateral ABCDABCD forms a parallelogram with ABCDAB \parallel CD and ADBCAD \parallel BC.

The height of this parallelogram (the distance between ABAB and CDCD) is the side length s=4s = 4 cm.

Thus, the area of the parallelogram ABCDABCD is: Area=base×height=(22)×4=8218 cm2\text{Area} = \text{base} \times \text{height} = (2\sqrt{2}) \times 4 = 8\sqrt{2} \approx 18 \text{ cm}^2

Conclusion

The area of quadrilateral ABCDABCD is approximately: 18 cm2\boxed{18 \text{ cm}^2}

So, the correct answer is D) 18 cm².

Would you like further details on any step?


Here are five related questions for further exploration:

  1. How would you calculate the face diagonal if only the surface area were given?
  2. What if AA and BB were not midpoints but quarter points on the edges—how would the area change?
  3. How does the area of a parallelogram depend on the angle between the sides?
  4. How can you find the volume of the cube given only the side length?
  5. What are other properties of a parallelogram that can help in calculating its area?

Tip: For any square or cube, knowing just one dimension (side length or surface area) allows you to calculate all other geometric properties.

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Math Problem Analysis

Mathematical Concepts

Geometry
Surface Area of Cube
Parallelogram Area
Distance Calculation in 3D

Formulas

Surface area of cube: 6s^2
Diagonal of a face: s√2
Area of parallelogram: base × height

Theorems

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Suitable Grade Level

Grades 9-12