Math Problem Statement

△ABC has sides AC = 3, BC = 5, and AB = 7. Each side is the diameter of a semicircle, and the areas of the grey figures are labeled S1, S2, and S3. What is S1 + S3 - S2?

Solution

The problem describes a triangle ABC\triangle ABC with sides AC=3AC = 3, BC=5BC = 5, and AB=7AB = 7. Each side is the diameter of a semicircle, and the shaded areas S1S_1, S2S_2, and S3S_3 correspond to these semicircles. The question asks for S1+S3S2S_1 + S_3 - S_2, where S1S_1, S2S_2, and S3S_3 are the areas of the grey semicircles as shown in the diagram.

Step 1: Area of a semicircle

The area AA of a semicircle is given by: A=12πr2A = \frac{1}{2} \pi r^2 where rr is the radius of the circle, which is half the diameter.

Step 2: Calculate the areas S1S_1, S2S_2, and S3S_3

The diameters of the semicircles are the sides of the triangle.

  • For AC=3AC = 3:

    • Radius = 32\frac{3}{2}
    • Area S1=12π(32)2=12π×94=9π8S_1 = \frac{1}{2} \pi \left(\frac{3}{2}\right)^2 = \frac{1}{2} \pi \times \frac{9}{4} = \frac{9\pi}{8}
  • For BC=5BC = 5:

    • Radius = 52\frac{5}{2}
    • Area S2=12π(52)2=12π×254=25π8S_2 = \frac{1}{2} \pi \left(\frac{5}{2}\right)^2 = \frac{1}{2} \pi \times \frac{25}{4} = \frac{25\pi}{8}
  • For AB=7AB = 7:

    • Radius = 72\frac{7}{2}
    • Area S3=12π(72)2=12π×494=49π8S_3 = \frac{1}{2} \pi \left(\frac{7}{2}\right)^2 = \frac{1}{2} \pi \times \frac{49}{4} = \frac{49\pi}{8}

Step 3: Compute S1+S3S2S_1 + S_3 - S_2

Now, we calculate: S1+S3S2=9π8+49π825π8S_1 + S_3 - S_2 = \frac{9\pi}{8} + \frac{49\pi}{8} - \frac{25\pi}{8} S1+S3S2=9π+49π25π8=33π8S_1 + S_3 - S_2 = \frac{9\pi + 49\pi - 25\pi}{8} = \frac{33\pi}{8}

Step 4: Compare with the options

The result 33π8\frac{33\pi}{8} is not directly one of the provided options. However, this might be a simplified or approximated form of one of the answers. After checking, it doesn't match exactly with the options, so there might be an alternative approach involving simplifying expressions with square roots from the triangle properties.

Let me know if you'd like further clarifications or if you'd like to explore other methods!


Do you want more details on solving this? Here are 5 related questions:

  1. How is the area of a full circle derived from the formula for a semicircle?
  2. Why is S2S_2 larger than both S1S_1 and S3S_3?
  3. How can the Pythagorean theorem apply to this triangle setup?
  4. What geometric properties of semicircles are used in this problem?
  5. Can this approach be extended to other polygons with similar configurations?

Tip: When dealing with semicircles, always halve both the radius and the area formula from a full circle to avoid calculation mistakes.

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Math Problem Analysis

Mathematical Concepts

Geometry
Semicircles
Areas of Figures
Triangles

Formulas

Area of a semicircle: A = 1/2 * π * r^2
Area difference involving semicircles
Radius is half of the side length

Theorems

Properties of Semicircles
Area Formulas

Suitable Grade Level

Grades 9-12