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In this lesson you will learn to find the circumference and area of a circle.
A circle is the set of all points in a plane that are equidistant from a given point, called the center.
A radius of a circle is a segment that joins the center of the circle to any point on the circle.
A diameter of a circle is a segment that contains the center of the circle and whose endpoints lie on the circle.
Recall that the perimeter of a figure is the distance around the figure. Well, circumference is just the perimeter of a circle. Like perimeter, the units of circumference are one-dimensional, like inches or centimeters.
The circumference of a circle is given by the formula C = 2πr, where π is the irrational number pi and r is the radius.
Since the diameter of a circle is twice the radius (d = 2r), the formula C = πd is an equivalent formula. You may choose which formula to use based on the information that you’re given.
What is the circumference of a circle of radius 8 m? Round your answer to the nearest meter.
Choice C is the correct answer. Since the radius of the circle is given, use the formula C = 2πr, where r = 8 m.
Recall that the area of a two-dimensional figure is the number of square units it contains. So, the area of a circle is given in units like in2 or ft2.
The area of a circle is given by the formula A = πr2, where r is the radius of the circle.
What is the area of this circle? Round your answer to the nearest square foot.
Choice B is correct. The area of the circle is given by the formula A = πr2. Since the diameter of the circle is 30 ft, the radius is 15 ft. If you answered C, you may have added p to the square of the radius rather than multiplying it. If you answered D, you may have used the diameter in the formula rather than the radius. So the area is:
