Buzz
1. Definition of the radius of a circle
In the formula for calculating the area of a circle: A = π .r2, you have become familiar with the concept of pi (π - a constant representing the ratio of the circumference to the diameter of a circle, π = 3.14 ) and the radius. So what is the radius of a circle?
- The radius is the distance from any point on the circle to the center of that circle.
- Radius = 1/2 of the diameter of the circle.
- The length of the radius is the same regardless of the direction measured.
- Symbol for radius: r.
Basic exercises finding the radius
Type 1: Given the diameter, calculate the radius.
- Apply the formula: D = 2.r (since the diameter is twice the radius).
=> r = D / 2.
Where: D is the diameter.
r is the radius.
Type 2: Given the circumference of the circle, calculate the radius.
- Apply the formula: C = 2. π. r.
=> r = C / (2π).
Where: C is the circumference of the circle.
r is the radius.
π = 3.14.
Some exercises applying basic problems to calculate the area of a circle with radius r
Problem statement:
1. Calculate the area of a circle with radius r as follows:
a) r = 4/5 cm
b) r = 12 dm
c) r = 0.4 m
d) r = 5 cm
2. Calculate the area of a circular dining table with a radius of 50 cm.
Instructions: We just need to apply the formula: A = π .r2
Solution
1. a) The area of the circle is: A = 3.14 x (4/5)2 = 2.0096 (cm2).
b) The area of the circle is: A = 3.14 x 122 = 452.16 (dm2).
c) The area of the circle is: A = 3.14 x (0.4)2 = 0.5024 (m2).
d) The area of the circle is: A = 3.14 x 52 = 78.5 (cm2).
2. The area of the dining table is:
3.14 x 502 = 7850 (cm2)
Answer: 7850 cm2.
Above, we have reinforced the knowledge of calculating the area of a circle with radius r for readers. We hope this will be useful knowledge for you.
In addition, you can also refer to how to calculate the area of a circle with diameter d, which is similar to calculating the area of a circle with radius r. Pay attention to follow along.
