CIRCLE

Learn some parts of a circle and formulae

Before we learn formulas for finding areas and circumference of circles, let’s discuss the following:

Consider the circle below:

Radius is a straight line from the center to any point on the circumference of a circle or sphere.

From the illustration above: the Radius is the line segment CO

Diameter: is a straight line joining two points on the circumference and passing through the centre.

From the illustration above: the Diameter is the line segment AB

Chord: is any straight line joining two points on the circumference.

From the illustration above: the Chord is the line segment DE

Note: A diameter is a chord passing through the centre.

A sector: is a portion of a circle enclosed by two radii.

From the illustration above: the Sector is the figure AOC

An arc: is a connected section of the circumference of a circle.

From the illustration above: the arcs are AC, CB, BE, ED and DA

A segment is a portion of a circle enclosed by a chord.

FORMULAE

Circle:

Area = πr² which means (π x r x r)

Circumference = πD which means π x D

Semi Circle:

Area = ¹/₂ πr² which means (¹/₂ x π x r x r)

Circumference = ¹/₂ πD + D

Three quarter circle

Area = ³/₄ πr² which means (³/₄ x π x r x r)

Circumference = ³/₄ πD + D

Quarter circle:

Area = ¹/₄ πr² which means (¹/₄ x π x r x r)

Circumference = ¹/₄ πD + D

Sector:

Area = ^θ/₃₆₀ πr² which means ( ^θ/₃₆₀ x π x r x r)

Circumference = ^θ/₃₆₀ πD + D

Circle with given angle:

Area = ^θ/₃₆₀ πr² which means ( ^θ/₃₆₀ x π x r x r)

Circumference = ^θ/₃₆₀ πD + D

************* END ********************