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Q12:
The circumference of a circle is $31.4$ cm. Find the radius and the area of the circle? (Take $\pi = 3.14$)

Solution :

Given Variables & Initial Setup

Let us define the fundamental parameters of the circle based on the provided data:

  • Circumference ($C$) = $31.4 \text{ cm}$
  • Mathematical constant ($\pi$) $\approx 3.14$
  • Radius ($r$) = ?
  • Area ($A$) = ?

Step 1: Deriving the Radius from the Circumference

[Per the geometric definition of a circle's circumference], the total arc length (perimeter) of a circle is directly proportional to its radius. The governing equation is:

$C = 2\pi r$

By substituting the given values into the equation, we can isolate the unknown variable, $r$:

$31.4 = 2 \times 3.14 \times r$

$31.4 = 6.28 \times r$

Dividing both sides by $6.28$ to solve for $r$:

$r = \frac{31.4}{6.28}$

$r = 5 \text{ cm}$

Step 2: Geometric Visualization

The following diagram illustrates the spatial relationship between the center, the calculated radius, and the given circumference of the circle.

O r = 5 cm C = 31.4 cm

Step 3: Calculating the Area of the Circle

[Per the fundamental theorem of circle geometry], the area $A$ enclosed by a circle is proportional to the square of its radius. The formula is expressed as:

$A = \pi r^2$

Substitute the derived radius ($r = 5 \text{ cm}$) and the given value of $\pi$ ($3.14$) into the area formula:

$A = 3.14 \times (5)^2$

First, evaluate the exponent:

$A = 3.14 \times 25$

Next, perform the multiplication to find the final area:

$A = 78.5 \text{ cm}^2$

Final Solution: The radius of the circle is $5 \text{ cm}$ and the area of the circle is $78.5 \text{ cm}^2$.


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