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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.
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 $78.5 \text{ cm}^2$.
More Questions from Class 9 Mathematics Coordinate Geometry EXERCISE 9.2
- Q1(a): Find the circumference of the circles with the following radius: (Take $\pi = \frac{22}{7}$) (a) $14$ cm
- Q1(b): Find the circumference of the circles with the following radius: (Take $\pi = \frac{22}{7}$) (b) $28$ mm
- Q1(c): Find the circumference of the circles with the following radius: (Take $\pi = \frac{22}{7}$) (c) $21$ cm
- Q10: From a circular card sheet of radius $14$ cm, two circles of radius $3.5$ cm and a rectangle of length $3$ cm and breadth $1$cm are removed. (as shown in the adjoining figure). Find the area of the remaining sheet. (Take $\pi = \frac{22}{7}$)
- Q11: A circle of radius $2$ cm is cut out from a square piece of an aluminium sheet of side $6$ cm. What is the area of the left over aluminium sheet? (Take $\pi = 3.14$)
- Q13: A circular flower bed is surrounded by a path $4$ m wide. The diameter of the flower bed is $66$ m. What is the area of this path? ($\pi = 3.14$)
- Q14: A circular flower garden has an area of $314$ m$^2$. A sprinkler at the centre of the garden can cover an area that has a radius of $12$ m. Will the sprinkler water the entire garden? (Take $\pi = 3.14$)
- Q15: Find the circumference of the inner and the outer circles, shown in the adjoining figure? (Take $\pi = 3.14$)
- Q16: How many times a wheel of radius $28$ cm must rotate to go $352$ m? (Take $\pi = \frac{22}{7}$)
- Q17: The minute hand of a circular clock is $15$ cm long. How far does the tip of the minute hand move in $1$ hour. (Take $\pi = 3.14$)
- Q2(a): Find the area of the following circles, given that: (a) radius = $14$ mm (Take $\pi = \frac{22}{7}$)
- Q2(b): Find the area of the following circles, given that: (b) diameter = $49$ m
- Q2(c): Find the area of the following circles, given that: (c) radius = $5$ cm
- Q3: If the circumference of a circular sheet is $154$ m, find its radius. Also find the area of the sheet. (Take $\pi = \frac{22}{7}$)
- Q4: A gardener wants to fence a circular garden of diameter $21$ m. Find the length of the rope he needs to purchase, if he makes $2$ rounds of fence. Also find the cost of the rope, if it costs ₹ $4$ per meter. (Take $\pi = \frac{22}{7}$)
- Q5: From a circular sheet of radius $4$ cm, a circle of radius $3$ cm is removed. Find the area of the remaining sheet. (Take $\pi = 3.14$)
- Q6: Saima wants to put a lace on the edge of a circular table cover of diameter $1.5$ m. Find the length of the lace required and also find its cost if one meter of the lace costs ₹ $15$. (Take $\pi = 3.14$)
- Q7: Find the perimeter of the adjoining figure, which is a semicircle including its diameter.
- Q8: Find the cost of polishing a circular table-top of diameter $1.6$ m, if the rate of polishing is ₹ $15/m^2$. (Take $\pi = 3.14$)
- Q9: Shazli took a wire of length $44$ cm and bent it into the shape of a circle. Find the radius of that circle. Also find its area. If the same wire is bent into the shape of a square, what will be the length of each of its sides? Which figure encloses more area, the circle or the square? (Take $\pi = \frac{22}{7}$)
CBSE Solutions for Class 9 Mathematics Coordinate Geometry
Chapters in CBSE - Class 9 Mathematics
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