Comprehension

A pot is made from a hollow sphere of inner radius 20 cm by cutting its upper portion horizontally. The height of the pot is 30 cm.

What is the inner radius of the circular opening of the pot so formed?

This question was previously asked in
UPSC CDS-I 2025 (Elementary Mathematics) Official Paper (Held On: 13 Apr, 2025)
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  1. \(10\sqrt{2}\) cm
  2. 15 cm 
  3. \(10\sqrt{3}\) cm
  4. 12 cm

Answer (Detailed Solution Below)

Option 3 : \(10\sqrt{3}\) cm
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Detailed Solution

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Given:

Inner radius of the hollow sphere (R) = 20 cm

Height of the pot (h) = 30 cm

Calculations:

Let 'r' be the inner radius of the circular opening of the pot.

Draw a vertical line from the center of the sphere to the center of the circular opening. This line segment will be perpendicular to the plane of the circular opening.

Let the center of the sphere be O. Let the center of the circular opening be C'.

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The radius of the sphere (R) goes from O to any point on the surface of the sphere, including the edge of the circular opening.

Consider a right-angled triangle formed by:

The radius of the sphere (R) as the hypotenuse, from the center of the sphere to any point on the edge of the circular opening.

d = |R - h| = |20 - 30| = |-10| = 10 cm.

Now, substitute the values into the Pythagorean theorem:

R2 = r2 + d2

202 = r2 + 102

400 = r2 + 100

r2 = 400 - 100

r2 = 300

r = \(\sqrt{300}\)

r = \(\sqrt{100 \times 3}\)

r = 10\(\sqrt{3}\) cm

∴ The inner radius of the circular opening of the pot is 10\(\sqrt{3}\) cm.

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