Circle vocabulary and labelled parts
Recognise and label all nine named circle parts, tracing the boundaries that distinguish an arc, a sector and a circular segment.
Exam scope and assessment notes
Circle properties links two shared Foundation rows and three additional Higher rows through nine original lessons; Higher includes the first three shared lessons. Use the actual named points, selected arcs and tangent rays, theorem reasons and positive ordered lengths. Keep requested values and derived marks hidden until every part of an explained attempt. A not-to-scale picture is not a measurement. Self-check comparisons award no marks or mastery. The no-formal-proof note belongs to H4.6C; it is not a blanket waiver for H4.6A or H4.7. Linear 4MA1 Issue 2 is checked against November 2026/June 2027 entry context; regional options depend on centre eligibility, and modular awards remain separate. Publisher links are navigation only: Book2 chapters5/8 carry literal circle tags, F4.6A has no literal tag, and no Book1 link is inferred. Chapter8 map326–349 versus proof chapter-start325 remains unresolved. Relevant final chapter bodies and Teaching Hub content are unverified. Construction, whole geometrical reasoning, trigonometry, mensuration and formal assessment remain separate; specialist approval is pending.
Before you begin
- Read labelled points, straight segments and shaded regions on a diagram.
- Use a length unit and multiply or divide by two.
- Distinguish an endpoint on a circle from its centre.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which description correctly identifies a sector of a circle?
What you will learn
- Recognise the centre, radius, chord, diameter, circumference, tangent, arc, sector and segment on labelled figures.
- Distinguish a curved boundary from a straight segment or shaded region.
- Use equal radii to relate a diameter to a radius without assuming every chord is a diameter.
Use the supplied points and givens. Amber marks retain your own selected points, boundaries and sweeps, including different choices.
Worked circle exploration
Use the three supplied copies of a circle, whose centre is O and labelled radius OA is 7 cm. Match and mark all nine terms: centre, radius, chord, diameter, circumference, tangent, arc, sector and segment. On the two shaded copies say which is the sector and which the segment, and trace each boundary. Give the diameter length. The circle is turned on the page; do not classify by page direction.
Original StudyForA circle exploration. Use the stated theorem hypotheses and givens; a visual resemblance is insufficient. These fixed comparisons do not establish full-book coverage, specialist approval or formal assessment validity.
Locate the centre and the straight segments
A circle consists of points at one fixed distance from its centre. A radius joins the centre to a point on the circle, so radii of the same circle have equal length. A chord joins two points on the circle. A diameter is a chord that passes through the centre; it contains two radii in a straight line and is twice their length.
Follow the actual endpoints when naming a segment. A line from the centre to the circumference is a radius even when it is tilted, while a chord can appear in any direction and need not pass through the centre. The picture’s centre must be given or correctly established: a point that merely looks central is not enough to identify a diameter.
Trace the circumference, arcs and tangent
The circumference is the entire curved boundary of the circle. An arc is a selected portion of that boundary between two endpoints. The same endpoints usually allow a shorter and a longer route around the circle, so trace or name the chosen arc rather than assuming that two endpoint letters always select one unambiguous portion.
A tangent is a straight line that touches a circle at one point. It is different from a secant, which crosses the circle at two points, and from a chord, which is the segment joining two circle points. Classify the touching line using its relationship to the circle, not whether the line appears vertical or horizontal on the page.
Identify the boundary of each shaded region
A sector is a region bounded by two radii and their selected arc. A circular segment is a region bounded by a chord and its selected arc. To decide which you see, trace every piece of the boundary: look for two straight edges meeting at the centre, or one chord joining the arc endpoints. An arc alone is only a curved boundary portion, not a shaded region.
Label the actual supplied figures and explain the distinction in words. Turning a circle or choosing a different size does not change these definitions. Here the aim is to recognise its parts; naming a sector does not by itself calculate an area, and naming an arc does not supply its length. Keep the stated length unit when using the radius–diameter relationship.
Sources for this lesson
Pause and explain
Which statement about a diameter is always true?
Worked example
Use the three supplied copies of a circle, whose centre is O and labelled radius OA is 7 cm. Match and mark all nine terms: centre, radius, chord, diameter, circumference, tangent, arc, sector and segment. On the two shaded copies say which is the sector and which the segment, and trace each boundary. Give the diameter length. The circle is turned on the page; do not classify by page direction.
Show the worked solution
- Locate O and distinguish a centre-to-circle radius from a chord whose two endpoints lie on the circle. AB is the chord passing through O, so it is a diameter.
- Use equal radii on each side of the centre: diameter AB=2×7=14 cm. A chord not passing through the centre is still a chord, but not a diameter.
- Trace the whole circumference versus the selected arc. Identify the tangent by its one contact with the circle, rather than by whether it appears vertical.
- Trace each shaded boundary: a sector uses two radii and the chosen arc; a circular segment uses a chord and the chosen arc. Label the actual regions, not only a list of words.
Answer O is the centre; OA a radius; AB a diameter of length 14 cm; CD a chord; the full curved boundary is the circumference; the touching line at A is a tangent; the selected curved CD portion is an arc. The two-radius-and-arc region is a sector; the chord-and-arc region is a segment.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Use the three supplied copies of a circle, whose centre is O and labelled radius OA is 5 cm. Match and mark all nine terms: centre, radius, chord, diameter, circumference, tangent, arc, sector and segment. On the two shaded copies say which is the sector and which the segment, and trace each boundary. Give the diameter length. The circle is turned on the page; do not classify by page direction.
Give me a hint
Trace the straight and curved pieces bounding each shaded copy before assigning a region name.
Compare my reasoning
- Locate O and distinguish a centre-to-circle radius from a chord whose two endpoints lie on the circle. AB is the chord passing through O, so it is a diameter.
- Use equal radii on each side of the centre: diameter AB=2×5=10 cm. A chord not passing through the centre is still a chord, but not a diameter.
- Trace the whole circumference versus the selected arc. Identify the tangent by its one contact with the circle, rather than by whether it appears vertical.
- Trace each shaded boundary: a sector uses two radii and the chosen arc; a circular segment uses a chord and the chosen arc. Label the actual regions, not only a list of words.
O is the centre; OA a radius; AB a diameter of length 10 cm; CD a chord; the full curved boundary is the circumference; the touching line at A is a tangent; the selected curved CD portion is an arc. The two-radius-and-arc region is a sector; the chord-and-arc region is a segment.
Look for these in your work
- Identify and mark all nine requested terms on the actual figures.
- Distinguish curved boundary, chord and shaded region; do not confuse sector with segment.
- Use two equal radii for the diameter and state centimetres.
Use the three supplied copies of a circle, whose centre is O and labelled radius OA is 9 cm. Match and mark all nine terms: centre, radius, chord, diameter, circumference, tangent, arc, sector and segment. On the two shaded copies say which is the sector and which the segment, and trace each boundary. Give the diameter length. The circle is turned on the page; do not classify by page direction.
Give me a hint
Use the actual marked centre and endpoints; turning the figure does not change any definition.
Compare my reasoning
- Locate O and distinguish a centre-to-circle radius from a chord whose two endpoints lie on the circle. AB is the chord passing through O, so it is a diameter.
- Use equal radii on each side of the centre: diameter AB=2×9=18 cm. A chord not passing through the centre is still a chord, but not a diameter.
- Trace the whole circumference versus the selected arc. Identify the tangent by its one contact with the circle, rather than by whether it appears vertical.
- Trace each shaded boundary: a sector uses two radii and the chosen arc; a circular segment uses a chord and the chosen arc. Label the actual regions, not only a list of words.
O is the centre; OA a radius; AB a diameter of length 18 cm; CD a chord; the full curved boundary is the circumference; the touching line at A is a tangent; the selected curved CD portion is an arc. The two-radius-and-arc region is a sector; the chord-and-arc region is a segment.
Look for these in your work
- Identify and mark all nine requested terms on the actual figures.
- Distinguish curved boundary, chord and shaded region; do not confuse sector with segment.
- Use two equal radii for the diameter and state centimetres.
Use the three supplied copies of a circle, whose centre is O and labelled radius OA is 6 cm. Match and mark all nine terms: centre, radius, chord, diameter, circumference, tangent, arc, sector and segment. On the two shaded copies say which is the sector and which the segment, and trace each boundary. Give the diameter length. The circle is turned on the page; do not classify by page direction.
Give me a hint
Mark each requested feature on the supplied copies, then relate diameter and radius without measuring the picture.
Compare my reasoning
- Locate O and distinguish a centre-to-circle radius from a chord whose two endpoints lie on the circle. AB is the chord passing through O, so it is a diameter.
- Use equal radii on each side of the centre: diameter AB=2×6=12 cm. A chord not passing through the centre is still a chord, but not a diameter.
- Trace the whole circumference versus the selected arc. Identify the tangent by its one contact with the circle, rather than by whether it appears vertical.
- Trace each shaded boundary: a sector uses two radii and the chosen arc; a circular segment uses a chord and the chosen arc. Label the actual regions, not only a list of words.
O is the centre; OA a radius; AB a diameter of length 12 cm; CD a chord; the full curved boundary is the circumference; the touching line at A is a tangent; the selected curved CD portion is an arc. The two-radius-and-arc region is a sector; the chord-and-arc region is a segment.
Look for these in your work
- Identify and mark all nine requested terms on the actual figures.
- Distinguish curved boundary, chord and shaded region; do not confuse sector with segment.
- Use two equal radii for the diameter and state centimetres.
Common mistakes
- Calling every chord a diameter without checking that it passes through the centre.
- Confusing the curved arc with a shaded segment, or the full circumference with one selected arc.
- Choosing sector or segment from its position or colour instead of tracing its boundary.
What can you explain now?
A straight segment joins two circumference points but misses the centre. Is it a chord, a diameter, a radius, or more than one of these?
Compare with the explanation
It is a chord, but not a diameter or a radius.
Both endpoints on the circle establish a chord. Missing the centre rules out a diameter, and neither endpoint is the centre as a radius requires.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Name all nine terms and describe the boundary of each. Explain why a diameter is a chord, why the reverse is not always true, and how to distinguish an arc from a segment.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.