Calculate regular-polygon interior and exterior angles
Calculate interior and exterior angles of regular convex polygons, including inverse side counts and impossible specifications.
Exam scope and assessment notes
Foundation and Higher share all seven section4.2 requirements A–G. The Higher table says See Foundation Tier; there are no additional H4.2 rows. These seven original lessons teach polygon names, convex/concave quadrilateral sums, all six named quadrilateral families, general polygon sums, regular interior/exterior angles and congruence correspondence. F:4.2C requires both the parallelogram/rectangle/rhombus/square lesson and the trapezium/kite lesson with substantive teaching, a worked example and every Guided, Independent and Transfer stage. F and G are two distinct published rows; the same substantive congruence lesson links both without inventing a third requirement. Seven lessons and28 selected seed cases are StudyForA’s design, not a syllabus-prescribed count. Rhombus/square diagonal endpoint-angle bisection, kite symmetry-diagonal endpoint-angle bisection and isosceles-trapezium equal diagonals are conservative standard-property breadth under the broad C wording, not new verbatim syllabus bullets or an assertion that the public book chart enumerates each one. Use labelled boundary order, actual interior/reflex arcs, given or justified parallel/equal-side/right-angle marks and geometric reasons before equations. Diagrams need not be to scale; do not measure them unless instructed. Use the local exactly-one-parallel-pair trapezium convention and different-length adjacent-pair kite fixtures; the sources do not settle every inclusive naming convention. Keep rectangle equal diagonals, rhombus perpendicular/bisected diagonals, square45° endpoint halves and kite one-way diagonal bisection distinct. General simple-polygon sums include checked concave triangulation; crossed boundaries, holes and curved outlines are outside these cases. Regular examples use consistent convex exterior turns and exact integer side-count checks; unknown-n full outlines follow the deduction, while impossible marked data remain condition cards. Congruence preserves full corresponding shape and size under turns/reflections, including reflex interiors; equal area, perimeter or angles alone is insufficient. A fixed reference picture and a correct number do not establish that a learner drew or justified the configuration. Written work is not automatically marked. Attempt acknowledgements and revealed references do not award marks or establish mastery. These structural links are not exhaustive subskill approval; specialist approval and formal assessment validation remain unestablished. Section4.3 symmetry,4.4 measurement/bearings,4.5 formal instrument constructions,4.11 similarity and the full H4.7A reasoning scope remain separate. Local symmetry-diagonal use is not complete symmetry coverage; no blanket formal-proof waiver is imported from unrelated circle-theorem notes. The wider Maths A course remains incomplete. Publisher book navigation is separate evidence: Book1 Shape and Space1 has literal F4.2B/C chapter and Hub19 Geometry of Shapes2 tags, not A; map60–99 and proof start60/next100 agree. Book2 Shape and Space6 has literal D/E chapter tags, with Hub59 Angles in Polygons2 for D and Hub58 Angles in Polygons1 for E; map128–160 and proof start128/next161 agree. The map associates F/G with Shape and Space7 pages223–248, while the public proof chapter starts222 and its next chapter starts248; proof contents instead place Congruent shapes129 and Similar shapes147 in Shape and Space6. Preserve that unresolved topic/page conflict rather than moving tags silently. Hub85 Similarity1 literally tags F4.2F and F4.11A, not G. A has no literal chapter/Hub tag and G has no literal Hub tag. Only selected Book1 public uncorrected proof body pages69–71 were inspected; Book2 relevant chapters are contents-only evidence in the public sample. Full final-book and Teaching Hub body/pagination alignment remains unverified; chapter links confer no full-book or modular teaching credit.
Before you begin
- Use the simple-polygon interior sum and straight-line angle relationship.
- Solve linear equations and divide exact numbers without inappropriate rounding.
- Distinguish one angle, a total and the selected exterior arc.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
A rectangle has four right angles but unequal adjacent side lengths. Is it regular?
What you will learn
- Use both equal sides and equal angles in the definition of a regular polygon.
- Relate each exterior turn to 360/n° and its adjacent interior to 180°.
- Solve inverse angle expressions and reject non-integer or too-small side counts.
Draw a regular nonagon, with nine boundary sides and equal-side/equal-angle marks. At one labelled vertex extend its incoming side past the vertex and mark the adjacent exterior turn and interior arc. Find one exterior, one interior and the total of all interior angles, checking the two ways of finding the total.
Draw or copy each labelled boundary on paper. Keep stated side lengths, selected interior/reflex/exterior arcs, parallel arrows and equality marks. Give geometric reasons before equations and substitute back. Exact givens determine deductions; screen diagrams are illustrative and must not be measured for an exact answer.
Regular polygon
- α1 · arc-1: unknown · equal-angle group 1
- α2 · arc-2: given ? · equal-angle group 1
- α3 · arc-3: unknown · equal-angle group 1
- α4 · arc-4: unknown · equal-angle group 1
- α5 · arc-5: unknown · equal-angle group 1
- α6 · arc-6: unknown · equal-angle group 1
- α7 · arc-7: unknown · equal-angle group 1
- α8 · arc-8: unknown · equal-angle group 1
- α9 · arc-9: unknown · equal-angle group 1
- α10 · exterior-B: given ?
Given regular: all boundary sides and all interior angles are equal. The selected exterior is on the actual side extension.
This separate check assumes an ordinary convex regular polygon. An angle label alone does not establish regularity.
Use both parts of regularity
A regular polygon has all its sides equal and all its interior angles equal. A non-square rhombus has equal sides but not equal angles; a non-square rectangle has equal angles but not equal sides. Neither is regular. A regular quadrilateral is a square because it satisfies both conditions.
The regular polygons considered here are ordinary convex ones with at least three sides. Draw equal-side ticks and equal-angle marks only where regularity is supplied. A name such as octagon gives its side count; regular octagon adds the equality information needed to divide totals equally.
Connect an exterior turn with its adjacent interior
Walk consistently around a convex polygon. At each vertex, extend the incoming side beyond the vertex and mark the turn from that continuation to the outgoing side. These one-per-vertex exterior turns make one complete 360° turn. In a regular n-gon they are equal, so each is 360/n°.
That exterior and its adjacent interior share a straight line, so their sum is 180°. Thus one regular interior is 180−360/n°. State which extension and arc you mean: an arbitrary outside reflex sweep is not the exterior turn used in this rule. As a check, multiplying the interior by n must agree with the total 180(n−2)°.
Recover and test the side count
If an interior is given, find its straight-line partner first; then n=360/exterior. If an interior and adjacent exterior are expressions, equate their sum to 180° before finding n. Check both the equation and the resulting angle values.
A valid ordinary regular polygon needs an integer n≥3. A fractional result cannot be rounded without changing the given angles. Begin an unknown-n task with the angle condition or a local adjacent-angle schematic; draw the full resolved polygon only after calculating n. A useful labelled sketch is not a claim to complete formal instrument constructions.
Sources for this lesson
Pause and explain
A regular convex polygon has interior angle 150°. What are its adjacent exterior turn and side count?
Worked example
Draw a regular nonagon, with nine boundary sides and equal-side/equal-angle marks. At one labelled vertex extend its incoming side past the vertex and mark the adjacent exterior turn and interior arc. Find one exterior, one interior and the total of all interior angles, checking the two ways of finding the total.
Show the worked solution
- A regular nonagon has n=9, and its equal exterior turns total 360°. One exterior is 360÷9=40°.
- The adjacent interior completes a straight line, so it is 180−40=140°. Label the 40° turn and 140° interior on their different arcs.
- The total is 180(9−2)=1260°. The independent check 9×140=1260° agrees; 1260° is the total, not one interior.
Answer Regular nonagon: exterior 40°, interior 140°, total interior angle 1260°. The exact turns sum to 360° and the adjacent pair to 180°.
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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.
Draw a regular octagon with eight boundary sides. At a named vertex extend the incoming side and mark the adjacent exterior turn and interior arc without selecting a reflex outside sweep. Calculate one exterior, one interior and the full interior total; annotate a second check.
Give me a hint
Find the equal exterior turn first, then use its straight-line partner and check the whole total.
Compare my reasoning
- With n=8, one exterior turn is 360÷8=45°. The consistent-turn rule refers to one chosen exterior at each vertex.
- Its adjacent interior is 180−45=135°. Keep the extension, 45° turn and 135° interior clearly distinguished.
- The total is 180(8−2)=1080°. Multiplying 8×135 also gives 1080°, checking both the single angle and the total.
Regular octagon: exterior 45°, interior 135°, total 1080°. Both 8×135 and 180(8−2) give that total.
Look for these in your work
- Mark the incoming-side extension and correct exterior turn.
- Calculate 45° exterior and 135° interior using named relationships.
- Check the 1080° total in two ways.
Begin with a local angle schematic, not a full resolved polygon. A regular convex polygon has an interior labelled (4x+12)° and its adjacent exterior turn labelled (x+3)°. Draw the shared straight-line relationship, find x and both angles, then infer the side count. Only after that deduction, draw or inspect a regular polygon with the resolved number of sides and check its interior total.
Give me a hint
Use the straight-line equation before dividing 360° by the recovered exterior.
Compare my reasoning
- The adjacent interior and exterior sum to 180°: (4x+12)+(x+3)=180, so 5x+15=180 and x=33.
- Substitute to get interior 144° and exterior 36°. Their sum is 180° and the exterior is the positive turn for a convex polygon.
- n=360÷36=10, a valid integer at least three. The full regular 10-gon can now be drawn as a check rather than used to reveal n in advance.
- Check the recovered ten-sided case using both total methods: 10×144=1440° agrees with 180(10−2)=1440°, so the one-angle and total-angle calculations are consistent.
x=33; interior 144°, exterior 36°, n=10 sides. Its total interior angle is 1440°. The full polygon is a check after the deduction.
Look for these in your work
- Keep unknown n hidden while forming and solving the adjacent-angle equation.
- Recover x=33,144°,36° and ten sides.
- Draw the resolved regular example afterwards and check its 1440° total.
A regular convex sign is specified by interior angle 156°. Begin from that condition and a local adjacent-angle schematic; find its exterior, number of sides and total interior angle before drawing the full resolved sign. A separate proposal specifies exterior turn 22°. Test whether that ordinary regular convex sign can exist. Keep the rejected proposal as a condition card rather than drawing a fictitious fractional-sided shape.
Give me a hint
For each specification, compute n exactly and test whether it is an integer at least three before drawing a resolved sign.
Compare my reasoning
- For the first sign, exterior=180−156=24°, so n=360÷24=15. This is a valid whole number of sides; draw the resolved 15-gon only after the deduction.
- Its total interior angle is 180(15−2)=2340°, also 15×156=2340°. Keep 156° as one angle, not the total.
- For the second proposal, n=360÷22=180/11. This is not an integer.
- Therefore no ordinary regular convex polygon has the specified 22° exterior turn. Do not round 180/11 or attach 22° labels to an invented regular outline.
For interior 156°: exterior 24°, n=15 and total 2340°. The 22° exterior proposal is impossible because n=180/11 is not a whole-number side count.
Look for these in your work
- Use the 156° interior to calculate the 24° exterior and 15 sides before drawing.
- Check the valid 2340° total.
- Reject the exact non-integer 180/11 without rounding or depicting an impossible regular polygon.
Common mistakes
- Calling a polygon regular from equal sides alone or equal angles alone.
- Using an arbitrary outside or reflex arc in place of the consistent exterior turn.
- Rounding a fractional side count, or showing a full resolved unknown-n polygon before making the deduction.
What can you explain now?
A condition card proposes an ordinary regular convex polygon with exterior turn 48°. Can it exist?
Compare with the explanation
No: n=360/48=15/2=7.5 is not an integer.
A side count must be an integer at least three. Rounding to seven or eight would change the required exterior angle, so the given specification is impossible.
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Make it stick
Define regularity using both equality conditions. Explain the consistent 360° exterior-turn rule, its 180° interior partner and the exact integer test in an inverse problem.
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.