Inscribed figure

In geometry, an inscribed figure is a geometric shape that is enclosed within another shape such that all vertices of the inner figure lie on the boundary of the outer shape, or the boundary of the outer shape is tangent to the inner figure, touching at designated points. This configuration ensures the inner shape fits precisely without crossing the outer boundary, forming a foundational concept in Euclidean geometry.^[1][2] The term encompasses two primary scenarios: a polygon inscribed in a circle, where all vertices of the polygon lie on the circle's circumference, making the circle the circumcircle of the polygon; and a circle inscribed in a polygon, known as the incircle, which is tangent to each side of the polygon at one point and centered at the incenter.^[2][3] These constructions are central to studying properties of cyclic quadrilaterals, where opposite angles sum to 180 degrees, and tangential quadrilaterals, which admit an incircle if the sums of the lengths of their opposite sides are equal.^[4] Notable theorems related to inscribed figures include the inscribed angle theorem, which posits that the measure of an inscribed angle is half the measure of the central angle subtending the same arc, and the fact that an angle inscribed in a semicircle is a right angle.^[5][6] These principles underpin applications in trigonometry, area calculations, and constructions, such as determining the radius of an incircle using the formula $r = A / s$r=A/s, where $A$A is the area and $s$s is the semiperimeter of the polygon.

Fundamental Concepts

Definition and Terminology

In geometry, an inscribed figure refers to a geometric shape A that is entirely contained within another shape B, with the vertices or key boundary points of A lying directly on the boundary of B.^[7][1] This configuration ensures that A fits snugly inside B without intersecting or extending beyond its boundary, emphasizing contact at discrete points rather than complete enclosure without touch. The terminology "inscribed" originates from classical Euclidean geometry, where it specifically describes a rectilinear figure (such as a polygon) placed within a circle such that each vertex of the figure lies on the circle's circumference. This usage, as defined in Euclid's Elements (Book IV, Definition 3), highlights the foundational role of inscription in ancient proofs involving cyclic figures.^[8] In modern contexts, the term extends beyond polygons and circles to general curves, but retains the core idea of boundary contact distinguishing it from merely internal figures, which are fully contained within B but may not touch its boundary at all.^[1] Understanding inscribed figures requires basic knowledge of primary geometric objects: polygons, which are closed two-dimensional shapes formed by three or more straight line segments connected end-to-end; circles, consisting of all points in a plane at a fixed distance (radius) from a central point; and curves, which represent continuous, often smooth paths that can bound regions without straight segments./01:_Points_Lines_Planes_and_Angles/1.04:_Polygons)/06:_Analytic_Geometry/6.01:_Lines) A representative example is a triangle inscribed in a circle, where the triangle's three vertices rest precisely on the circle's circumference, ensuring the triangle lies entirely inside the circle. In such a diagram, the circle appears as a rounded enclosure with the triangle's straight sides connecting the contact points, illustrating the inscription without any overlap or protrusion. This setup contrasts briefly with circumscription, where the outer figure's boundary touches the inner figure externally.^[9]

Distinction from Circumscription

In geometry, circumscription refers to a configuration where a figure B is drawn around another figure A such that the boundary of B passes through all vertices or key points of A, with A entirely contained within B.^[9] This setup positions B as the enclosing shape, touching A at its extremities from the outside.^[10] The primary distinction between inscription and circumscription lies in their relational perspectives and contact dynamics: inscription involves an inner figure whose vertices or boundary points lie on the boundary of an outer figure, with the inner shape touching the outer from within; in contrast, circumscription involves an outer figure whose boundary passes through the vertices or key points of an inner figure, touching it from without./02:_General_Triangles/2.05:_Circumscribed_and_Inscribed_Circles) These concepts are inverses of each other, often describing the same geometric pair depending on which figure is viewed as inner or outer—for instance, a polygon inscribed in a circle is equivalently a circle circumscribed around the polygon.^[10] To illustrate this duality, consider diagrams of a triangle and a circle: one shows the triangle inside the circle with its vertices on the circle's boundary (inscription of the triangle), while the dual depicts the circle inside the triangle tangent to its sides (circumscription of the triangle around the circle), highlighting how the roles reverse based on the chosen reference figure.^[11] Such paired visuals clarify the symmetric yet perspective-dependent nature of these terms, as seen in standard geometric illustrations of polygonal inscription.^[12] Etymologically, "inscribed" derives from the Latin inscribere, meaning "to write or draw in" or "to inscribe within," emphasizing the act of placing one figure inside another; whereas "circumscribed" stems from circumscribere, meaning "to draw around" or "to encircle," underscoring the enclosing action.^[7][13] This linguistic contrast reinforces the conceptual inverse, with roots tracing back to classical Latin usage in geometric contexts.^[14]

Types of Inscribed Figures

Inscribed Polygons

An inscribed polygon is a polygonal figure whose vertices lie on the boundary of an enclosing geometric shape, with the entire polygon contained within or on that boundary.^[10] The primary case involves a polygon inscribed in a circle, known as a cyclic polygon, where all vertices lie on the circumference of the circle. Every triangle is cyclic, as it always admits a circumcircle passing through its three vertices, but higher-sided polygons are cyclic only if their vertices are concyclic.^[15] Secondary cases include a polygon inscribed in another polygon, such as a triangle positioned inside a quadrilateral with each vertex of the triangle lying on a different side of the quadrilateral. In this configuration, the inner polygon's vertices contact the outer polygon's boundary without the inner figure extending beyond it.^[16] For any inscription, all vertices of the inner polygon must coincide with the boundary of the outer figure, and no part of the inner polygon, including its edges and interior, may lie outside the outer figure. This ensures the inner polygon is fully enclosed while maintaining contact at the vertices.^[10] A representative example is the regular pentagon inscribed in a circle, which can be constructed using a compass and straightedge as follows:

1. Draw a diameter through the center O of the circle, marking endpoints C and M.
2. Construct a perpendicular to this diameter at O, marking point S where it intersects the circle.
3. Find the midpoint L of segment SO.
4. With the compass centered at L and radius equal to LS (or LO), draw a smaller circle.
5. Draw the line from M through L, intersecting the smaller circle at points N and P.
6. With the compass centered at M and radius MP, draw an arc intersecting the original circle at points A and E.
7. With the compass centered at M and radius MN, draw an arc intersecting the original circle at points B and D.
8. Connect the points A, B, C, D, and E in order to form the pentagon.^[17]

Inscribed Circles and Curves

An incircle, also known as the inscribed circle, is a circle tangent to each side of a polygon, with its center known as the incenter.^[18] A polygon that admits such an incircle is termed tangential or inscriptable, meaning it possesses the necessary geometric properties to support a circle touching all its sides internally.^[19] Beyond circles, general inscribed curves include non-circular shapes like ellipses that fit within polygons such as quadrilaterals, where the curve is tangent to each side at designated points.^[20] For instance, an inellipse in a quadrilateral touches all four sides, with its center lying along the line segment connecting the midpoints of the diagonals.^[20] These curves maintain contact with the polygon's boundary while remaining entirely within the figure. The fundamental condition for any inscribed curve is that it must be tangent to the inner boundary of the enclosing figure at multiple specified points, ensuring no intersection or crossing of the boundary, which preserves the curve's interior position.^[18] This tangency requirement distinguishes inscribed curves from other internal figures, emphasizing smooth contact rather than discrete vertex placement as seen in inscribed polygons. A representative example is the incircle of an equilateral triangle, where the circle touches each of the three equal sides at their midpoints due to the triangle's symmetry, providing an intuitive sense of balanced tangency that maximizes the circle's size within the bounded space.^[21] The radius of this incircle scales proportionally with the side length, offering a clear illustration of how symmetry influences the points of contact.^[21]

Geometric Properties

Contact and Tangency Conditions

In inscribed polygons, the vertices lie precisely on the boundary of the enclosing curve or lines, ensuring the polygon is fully contained without crossing the boundary. This vertex contact defines the inscription, where each corner point coincides exactly with a point on the outer shape's perimeter.^[22] For inscribed circles within polygons, known as incircles, the circle is tangent to each side of the polygon at exactly one point, maximizing the circle's size while remaining inside. A key property arises from the tangent segments theorem: the lengths of the tangent segments from each vertex to the points of tangency on the adjacent sides are equal, which is a direct consequence of equal tangents drawn from an external point to a circle.^[23] In general, contact conditions for inscribed figures require orthogonality between the radius (or normal) at the contact point and the tangent line to the boundary, ensuring the inner figure touches without penetrating. Additionally, there is no intersection between the inscribed figure and the enclosing boundary beyond these isolated contact points, maintaining separation elsewhere.^[24] For smooth curves, tangency conditions can be established conceptually through local analysis: at the contact point, the curves must share the same position and first derivative (tangent vector), with higher-order derivative matching determining the order of contact; this follows from the definition of differentiability for parametrized curves, where equal slopes prevent crossing./03%3A_Topics_in_Differential_Calculus/3.01%3A_Tangent_Lines)

Symmetry and Regularity

Inscribed regular polygons, also known as cyclic regular polygons, exhibit rotational symmetry that aligns with the inherent symmetry of the circumscribed circle. Specifically, a regular n-gon inscribed in a circle possesses n-fold rotational symmetry, meaning it can be rotated by multiples of 360°/n around the circle's center to map onto itself, mirroring the circle's continuous rotational invariance.^[25] This symmetry arises because the vertices are equally spaced on the circle, ensuring that each rotation preserves the figure's structure. For instance, a square inscribed in a circle demonstrates 90-degree rotational symmetry, allowing it to coincide with itself after quarter-turn rotations, which enhances the overall central symmetry of the configuration.^[25] A polygon qualifies as regular when it is both equilateral—all sides of equal length—and equiangular—all interior angles equal—while being inscribed in a circle, which guarantees its vertices lie on the circumference.^[26] This inscription enforces the necessary uniformity, as the equal chord lengths corresponding to equal central angles ensure equilateral sides, and the consistent angular spacing produces equiangular vertices. The process of inscription thus preserves and amplifies the central symmetry inherent in regular polygons, particularly for even-sided ones where 180-degree rotations (point symmetry) are possible, creating a balanced, invariant figure under group transformations like the dihedral group.^[25] In contrast, non-regular polygons inscribed in circles, such as irregular cyclic quadrilaterals, maintain the basic property of having vertices on a common circle but lack the full rotational and reflectional symmetry of their regular counterparts. These figures satisfy specific cyclic conditions, including supplementary opposite angles (summing to 180°), yet they do not exhibit uniform side lengths or angles, resulting in asymmetric distortions that break higher-order invariances.^[27] For example, an irregular cyclic quadrilateral may have bilateral symmetry along a single axis but fails to achieve the multi-fold rotational symmetry seen in regular inscribed polygons, highlighting how inscription alone does not impose regularity without additional equilateral and equiangular constraints.^[27]

Theorems and Formulas

Inscribed Angle Theorem

The inscribed angle theorem states that the measure of an inscribed angle in a circle is half the measure of the central angle that subtends the same arc.^[28] This theorem applies to an inscribed angle formed by two chords sharing a common endpoint on the circle's circumference, intercepting a specific arc.^[29] Mathematically, if $\theta_{\text{inscribed}}$θinscribed​ denotes the measure of the inscribed angle and $\theta_{\text{central}}$θcentral​ denotes the measure of the central angle subtending the same arc, then $$\theta_{\text{inscribed}} = \frac{1}{2} \theta_{\text{central}},$$θinscribed​=21​θcentral​, where the angles are expressed in degrees or radians.^[30] A standard proof relies on properties of isosceles triangles and considers three cases based on the position of the vertex of the inscribed angle relative to the central angle. Consider a circle with center $O$O and points $A$A, $B$B, $C$C on the circumference, where $\angle ABC$∠ABC is the inscribed angle subtending arc $AC$AC, and $\angle AOC$∠AOC is the central angle. Draw radii $OA$OA, $OB$OB, and $OC$OC.

• Case 1: One ray of the inscribed angle (say, AB) is a diameter. Triangles $OAB$OAB and $OCB$OCB are considered, but fundamentally, since AB is diameter, $\angle ACB = 90^\circ$∠ACB=90∘ in the semicircle, and extensions apply.
• More generally, draw the diameter through B if needed. For B on the major or minor arc: In $\triangle AOB$△AOB, $OA = OB$OA=OB so base angles $\angle OAB = \angle OBA$∠OAB=∠OBA; similarly in $\triangle BOC$△BOC, $\angle OBC = \angle OCB$∠OBC=∠OCB. The inscribed angle $\angle ABC = \angle OBA + \angle OBC$∠ABC=∠OBA+∠OBC. The central angle $\angle AOC = \angle AOB + \angle BOC$∠AOC=∠AOB+∠BOC, where $\angle AOB = 180^\circ - 2\angle OBA$∠AOB=180∘−2∠OBA and $\angle BOC = 180^\circ - 2\angle OBC$∠BOC=180∘−2∠OBC, leading to $\angle AOC = 360^\circ - 2(\angle OBA + \angle OBC) = 360^\circ - 2\angle ABC$∠AOC=360∘−2(∠OBA+∠OBC)=360∘−2∠ABC. Adjusting for the arc (minor arc case yields the half relation directly via exterior angle or case division). Full rigor uses diameter extension: Let D be the other end of diameter through B; then $\angle ABD = \angle CBD = 90^\circ$∠ABD=∠CBD=90∘ or apply inscribed in semicircle, and add/subtract: $\angle ABC = \frac{1}{2} (\angle AOD \pm \angle COD)$∠ABC=21​(∠AOD±∠COD), equating to $\frac{1}{2} \angle AOC$21​∠AOC.^[31][28]

Key corollaries follow directly. All inscribed angles subtending the same arc are equal, as each is half the fixed central angle for that arc.^[30] Additionally, an angle inscribed in a semicircle is a right angle, since the central angle subtending a semicircular arc is $180^\circ$180∘, yielding $90^\circ$90∘.^[29] The theorem extends to applications in cyclic quadrilaterals, where a quadrilateral inscribed in a circle has opposite angles summing to $180^\circ$180∘. This follows because each pair of opposite angles subtends complementary arcs that together form the full circle, so their measures are half of arcs summing to $360^\circ$360∘.^[32]

Area and Length Relations

Inscribed polygons within circles exhibit specific area relations derived from their geometric symmetry. For a regular n-gon inscribed in a circle of radius $r$r, the area $A$A is given by $$A = \frac{1}{2} n r^2 \sin\left(\frac{2\pi}{n}\right),$$A=21​nr2sin(n2π​), where the formula arises from dividing the polygon into $n$n isosceles triangles, each with two sides of length $r$r and a central angle of $2\pi/n$2π/n.^[33] This expression quantifies how the polygon's area approaches that of the circle, $\pi r^2$πr2, as $n$n increases. Conversely, for a circle inscribed in a tangential polygon (one admitting an incircle tangent to all sides), the inradius $r$r relates the polygon's area $A$A to its semiperimeter $s$s via $r = A / s$r=A/s, or equivalently, $A = r s$A=rs.^[34] The area of the incircle itself is then $\pi r^2$πr2. The semiperimeter $s$s equals half the perimeter $p$p, so $p = 2s$p=2s, and this perimeter links directly to the tangency conditions, where $s$s represents the total length of the tangent segments from the vertices to the points of tangency.^[35] These relations can be derived by partitioning the figure into sectors or triangles. For an inscribed regular polygon, the area formula follows from summing the areas of $n$n triangular sectors from the circle's center, each with area $\frac{1}{2} r^2 \sin(2\pi/n)$21​r2sin(2π/n). For the incircle case, the polygon's area decomposes into $n$n triangles from the incenter to each side, each with height $r$r and base equal to the side length, yielding $A = r \sum (\text{side lengths}/2) = r s$A=r∑(side lengths/2)=rs. For more general inscribed curves, such as smooth curves tangent to a boundary, analogous relations emerge via integration over arc lengths and radial distances, though explicit forms depend on the curve's parametrization.

Applications and Examples

In Polygonal and Circular Contexts

One prominent example of an inscribed figure is the equilateral triangle inscribed in a circle, where the three vertices lie on the circumference and are equally spaced, dividing the circle into three congruent 120-degree arcs. This placement results in the circle's center coinciding with the triangle's centroid, circumcenter, orthocenter, and incenter, imparting a high degree of symmetry and visual balance to the figure.^[36][37] In contrast, an incircle inscribed in a square touches all four sides at their midpoints, creating four points of tangency that form a smaller square rotated 45 degrees relative to the original. From each vertex of the square, the two tangent segments to the adjacent points of tangency are equal in length, each measuring half the side length of the square, which underscores the uniformity of the inscription.^[38][5] When inscribing a non-equilateral triangle in a circle, the vertices are positioned such that the arcs between them correspond to twice the opposite angles, resulting in unequal arc lengths and an asymmetric arrangement relative to the center, unlike the uniform symmetry of the equilateral case; this can alter the perceived proportions and regularity of the triangle's shape.^[39] In ancient Greek geometry, such inscribed figures, particularly regular polygons in circles, were employed by Archimedes to approximate the circumference and area of a circle by computing the perimeters of inscribed and circumscribed polygons with increasing numbers of sides, providing early bounds for the value of π.^[40]

Advanced Geometric Constructions

In geometric constructions, inscribed polygons provide a method to divide a circle into equal parts using only a compass and straightedge. For instance, a regular hexagon can be inscribed in a given circle by constructing six equilateral triangles that share the circle's center as a vertex, where each side of the hexagon equals the radius of the circle.^[41] This construction, dating back to ancient geometry, relies on the property that arcs subtended by equal central angles are equal, ensuring the hexagon's equilateral and equiangular nature.^[41] Inscribed figures also play a key role in proofs involving concurrency within triangles. The incircle, tangent to all three sides, has its center at the incenter, which is the point of concurrency for the triangle's angle bisectors; this concurrency follows from the equal perpendicular distances from the incenter to the sides, as established by the tangency conditions.^[42] Such proofs demonstrate how the incircle's position verifies the intersection of bisectors at a single point equidistant from the sides.^[42] Extensions to conic sections involve inscribed ellipses in triangles, particularly for maximizing enclosed area. The Steiner inellipse, tangent to the sides at their midpoints, achieves the maximum area among all ellipses inscribed in a given triangle, with its area equal to $\frac{\pi}{3\sqrt{3}}$33

​π​ times the triangle's area in certain normalized cases.^[43] This ellipse arises from the affine transformation properties of the triangle, preserving the maximality under such mappings.^[43] In modern applications, inscribed polygons approximate circles in computer graphics rendering, where regular polygons with increasing sidesides efficiently model curved boundaries to leverage hardware polygon rasterization while minimizing visual artifacts.^[44]

References

1. https://mathworld.wolfram.com/Inscribed.html
2. http://aleph0.clarku.edu/~djoyce/elements/bookIV/defIV.html
3. https://sites.uni.edu/theron/assets/EG_main12.pdf
4. https://mathworld.wolfram.com/TangentialQuadrilateral.html
5. https://www.cerritos.edu/dford/SitePages/Math_70_F13/CircleDefinitionsandTheorems.pdf
6. https://espace.bsu.edu/aarc/wp-content/uploads/sites/132/2016/07/Lesson-Plan-Inscribed-Angles.pdf
7. https://www.mathopenref.com/inscribed.html
8. https://farside.ph.utexas.edu/Books/Euclid/Elements.pdf
9. https://mathworld.wolfram.com/Circumscribed.html
10. https://www.geeksforgeeks.org/maths/inscribed-and-circumscribed-figures/
11. https://mathmonks.com/circle/inscribed-and-circumscribed-circles
12. https://opencurriculum.org/5492/circumscribed-and-inscribed-circles/
13. https://www.etymonline.com/word/circumscribe
14. https://www.vocabulary.com/dictionary/circumscribe
15. https://mathworld.wolfram.com/CyclicPolygon.html
16. https://math.stackexchange.com/questions/4492467/inscribing-one-regular-polygon-in-another
17. https://www.mathopenref.com/printinpentagon.html
18. https://mathworld.wolfram.com/Incircle.html
19. https://mathworld.wolfram.com/Inradius.html
20. https://mathworld.wolfram.com/Inellipse.html
21. https://mathworld.wolfram.com/EquilateralTriangle.html
22. https://flexbooks.ck12.org/cbook/ck-12-basic-geometry-concepts/section/9.6/primary/lesson/inscribed-quadrilaterals-in-circles-bsc-geom/
23. https://www.mathopenref.com/polygonincircle.html
24. https://flexbooks.ck12.org/cbook/ck-12-cbse-math-class-10/section/10.2/primary/lesson/tangent-to-a-circle/
25. https://www.andrews.edu/~calkins/math/webtexts/geom06.htm
26. https://mathresearch.utsa.edu/wiki/index.php?title=Definition_of_Polygons
27. https://www.math.uakron.edu/amc/Geometry/HS_MSGeometryLessons/6.3inscribedquadsandllogram.pdf
28. https://www.mcs.uvawise.edu/msh3e/resources/geometryBook/16Circles.pdf
29. https://www.cerritos.edu/dford/SitePages/Math_70_F13/Math70_F2013_Lecture-Sections6-1thru6-2.pdf
30. http://www.geom.uiuc.edu/~dwiggins/conj44.html
31. https://www.khanacademy.org/math/geometry/hs-geo-circles/hs-geo-inscribed-angles/a/inscribed-and-central-angles-proof
32. http://www.geom.uiuc.edu/~dwiggins/conj47.html
33. https://mathworld.wolfram.com/RegularPolygon.html
34. https://math.stackexchange.com/questions/625391/relation-between-the-radius-and-the-area-of-tangential-polygon
35. https://www.atm.org.uk/write/MediaUploads/Journals/MT241/Non-Member/MT241-14-02-EXTRA.pdf
36. http://aleph0.clarku.edu/~djoyce/elements/bookIV/propIV11.html
37. https://mathed.miamioh.edu/index.php/ggbj/article/download/86/97
38. https://circles.math.ucla.edu/circles/lib/data/Handout-3808-3423.pdf
39. https://people.math.harvard.edu/~knill/teaching/mathe320_2011/handouts/02-geometry.pdf
40. https://www.exploratorium.edu/pi/history-of-pi
41. http://aleph0.clarku.edu/~djoyce/elements/bookIV/propIV15.html
42. https://www.ms.uky.edu/~droyster/courses/spring08/math6118/Classnotes/Chapter04.pdf
43. https://mathworld.wolfram.com/SteinerInellipse.html
44. https://web.wlu.ca/science/physcomp/ikotsireas/CP411_notes/HillCh6.pdf