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Circle Basics

Parts of a circle and basic properties

Written and reviewed by the Study Mondo Education TeamLast updated
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Circle Basics

Definitions

Circle: The set of all points equidistant from a center point.

Radius: Distance from center to any point on the circle (symbol: rr)

Diameter: Distance across circle through center (symbol: dd) d=2rd = 2r

Chord: Line segment connecting two points on the circle

Secant: A line that intersects the circle at two points

Tangent: A line that touches the circle at exactly one point

Key Properties

Tangent Property: A tangent line is perpendicular to the radius at the point of tangency.

Chord Property: A perpendicular from the center to a chord bisects the chord.

Equal Chords: Chords equidistant from the center are congruent.

Circumference

The distance around a circle: C=2πr=πdC = 2\pi r = \pi d

Area

A=πr2A = \pi r^2

Arc Length

For a central angle of θ\theta degrees: Arc length=θ360°×2πr\text{Arc length} = \frac{\theta}{360°} \times 2\pi r

📚 Practice Problems

1Problem 1easy

❓ Question:

A circle has a radius of 5. Find the circumference and area.

💡 Show Solution

Circumference: C=2πr=2π(5)=10πC = 2\pi r = 2\pi(5) = 10\pi

Area: A=πr2=π(5)2=25πA = \pi r^2 = \pi(5)^2 = 25\pi

Answer: Circumference = 10π10\pi (or ≈ 31.4), Area = 25π25\pi (or ≈ 78.5)

2Problem 2medium

❓ Question:

A circle has diameter 16. Find the length of an arc with central angle 45°45°.

💡 Show Solution

Step 1: Find the radius r=d2=162=8r = \frac{d}{2} = \frac{16}{2} = 8

Step 2: Use arc length formula Arc length=θ360°×2πr\text{Arc length} = \frac{\theta}{360°} \times 2\pi r

=45360×2π(8)= \frac{45}{360} \times 2\pi(8)

=18×16π= \frac{1}{8} \times 16\pi

=2π= 2\pi

Answer: Arc length is 2π2\pi (or ≈ 6.28)

3Problem 3hard

❓ Question:

A chord is 8 cm from the center of a circle with radius 10 cm. Find the length of the chord.

💡 Show Solution

Draw a radius to the chord's endpoint and a perpendicular from center to chord.

This creates a right triangle:

  • Hypotenuse = radius = 10
  • One leg = distance from center = 8
  • Other leg = half the chord length

Use Pythagorean Theorem: 82+(c2)2=1028^2 + \left(\frac{c}{2}\right)^2 = 10^2

64+c24=10064 + \frac{c^2}{4} = 100

c24=36\frac{c^2}{4} = 36

c2=144c^2 = 144

c=12c = 12

Answer: The chord length is 12 cm

Explain using:

📌 Related Topics in Circles

❓ Frequently Asked Questions

What is Circle Basics?▾
Parts of a circle and basic properties
How can I study Circle Basics effectively?▾
Start by reading the study notes and working through the examples on this page. Then use the flashcards to test your recall. Practice with the 3 problems provided, checking solutions as you go. Regular review and active practice are key to retention.
Is this Circle Basics study guide free?▾
Yes — all study notes, flashcards, and practice problems for Circle Basics on Study Mondo are free to access. No account is needed.
What course covers Circle Basics?▾
Circle Basics is part of the Geometry course on Study Mondo, specifically in the Circles section. You can explore the full course for more related topics and practice resources.
Are there practice problems for Circle Basics?▾
Yes, this page includes 3 practice problems with detailed solutions. Each problem includes a step-by-step explanation to help you understand the approach.