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Mathematics LibreTexts

6.2.2: Circles

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6.2.2 Learning Objectives

  • Identify the radius and diameter of a circle.
  • Use π or its approximate value in formulas.
  • Find the circumference of a circle.
  • Find the area of a circle.
  • Find the perimeter or area of composite figures involving circles/

Circumference/Diameter/Radius

Definition: Part of a Circle

The circumference of a circle is the distance around the circle.

A diameter of a circle is any line segment that passes through the center of the circle and has its endpoints on the circle.

A radius of a circle is any line segment having as its endpoints the center of the circle and a point on the circle.
The radius is one half the diameter.

A circle with a line directly through the middle, ending at the edges of the shape. The entire length of the line is labeled diameter, and the length of the portion of the line from the center of the circle to the edge of the circle is labeled radius.

 

The Number π

The symbol π, read "pi," represents the nonterminating, nonrepeating decimal number 3.14159 … . This number has been computed to millions of decimal places without the appearance of a repeating block of digits.

For computational purposes, π is often approximated as 3.14. We will write π3.14 to denote that π is approximately equal to 3.14. The symbol "≈" means "approximately equal to." Use the π key on your calculator when evaluating for the best approximation.

Formulas

To find the circumference of a circle, we need only know its diameter or radius. We then use a formula for computing the circumference of the circle.

Definition: Formula

A formula is a rule or method for performing a task. In mathematics, a formula is a rule that directs us in computations.

Formulas are usually composed of letters that represent important, but possibly unknown, quantities.

If C,d, and r represent, respectively, the circumference, diameter, and radius of a circle, then the following two formulas give us directions for computing the circum­ference of the circle.

Circumference Formulas

  1. C=πd
  2. C=2πr

Example 1

Find the exact circumference of the circle.

A circle with a dashed line from one edge to the other, labeled d = 7 in.

Solution

Use the formula C=πd.

C=π7 in

By commutativity of multiplication,

C=7 inπ

C=7π in, exactly

This result is exact since π has not been approximated.

Example 2

Find the approximate circumference of the circle.

A circle with a dashed line from one edge to the other, labeled d = 6.2 mm.

Solution

Use the formula C=πd.

C(3.14)(6.2)

C19.648 mm

This result is approximate since π has been approximated by 3.14.

Example 3

Find the approximate circumference of a circle with radius 18 inches.

Solution

Since we're given that the radius, r, is 18 in, we'll use the formula C=2πr.

C(2)(3.14)(18 in)

C113.04 in

Example 4

Find the approximate area of the figure.

A cane-shaped object of an even thickness, with one straight portion and one portion shaped in a half-circle. The thickness is 2.0cm, the length of the straight portion is 5.1cm, and the radius of the semicircle portion is 6.2cm.

Solution

We notice that we have two semicircles (half circles).

The larger radius is 6.2 cm.

The smaller radius is 6.2 cm - 2.0 cm = 4.2 cm.

The width of the bottom part of the rectangle is 2.0 cm.

Perimeter=2.0 cm5.1 cm2.0 cm5.1 cm(0.5)(2)(3.14)(6.2 cm)Circumference of outer semicircle.  +(0.5)(2)(3.14)(4.2 cm)_Circumference of inner semicircle.6.2 cm - 2.0 cm = 4.2 cmThe 0.5 appears because we want theperimeter of only half a circle.

Perimeter2.0 cm5.1 cm2.0 cm5.1 cm19.468 cm+13.188 cm_48.856 cm

Try It Now 1

Find the exact circumference of the circle.

A circle with a line through the middle, ending at the edges of the circle. The line is labeled, d = 9.1in.

Answer

9.1π in

Try It Now 2

Find the approximate circumference of the circle.

A circle with a line through the middle, ending at the edges of the circle. The line is labeled, d = 1.8in.

Answer

5.652 mm

Try It Now 3

Find the approximate circumference of the circle with radius 20.1 m.

Answer

126.228 m

Try It Now 4

Find the approximate outside perimeter of

A shape best visualized as a hollow half-circle. The thickness is 1.8mm, and the diameter of the widest portion of the half-circle is 16.2mm.

Answer

41.634 mm

 

Area of a Circle

  Figure Area Formula Statement
A circle. Circle Ac=πr2 Area of a circle is π times the square of the radius.

Example 5

Find the approximate area of the circle.

A circle with radius 16.8ft.

Solution

Ac=πr2(3.14)(16.8 ft)2(3.14)(282.24 sq ft)888.23 sq ft

The area of this circle is approximately 886.23 sq ft.

Example 6

Find the approximate area of the circle.

A circle with a dashed line from one edge to the other, labeled d = 6.2 mm.

Solution

In this case we are given the diameter instead of the radius. We need to find the radius before we can calculate the area.

r=d2=6.22=3.1mm

Ac=πr2(3.14)(6.2 ft)2(3.14)(38.44 sq mm)120.7 sq mm

The area of this circle is approximately 120.7 sq mm.

Example 4

Find the approximate area of the figure.

A cane-shaped object of an even thickness, with one straight portion and one portion shaped in a half-circle. The thickness is 2.0cm, the length of the straight portion is 5.1cm, and the radius of the semicircle portion is 6.2cm.

Solution

We notice that we have two semicircles (half circles) and a small rectangle

The larger radius is 6.2 cm.

The smaller radius is 6.2 cm - 2.0 cm = 4.2 cm.

The width of the bottom part of the rectangle is 2.0 cm.

The area of this shape will be A=area of the rectangle+area of the outer semicirclearea of the inner semicircle

Area of the rectangle: A=5.12=10.2 cm

Area of the outer semicircle A=π(6.2)22=9.7 cm

Area of the inner semicircle A=π(4.2)22=6.6 cm

A=area of the rectangle+area of the outer semicirclearea of the inner semicircle=10.2cm+9.7cm6.6cm=13.3cm


This page titled 6.2.2: Circles is shared under a not declared license and was authored, remixed, and/or curated by Leah Griffith, Veronica Holbrook, Johnny Johnson & Nancy Garcia.

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