Use quadrilateral angle sums in convex and concave diagrams
Use the 360° interior-angle sum with named quadrilateral vertices, equations and correctly selected reflex interiors.
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 triangle, straight-line and full-turn angle sums from section 4.1.
- Solve a linear equation and substitute into an expression.
- Identify a polygon boundary and a named angle’s middle vertex.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
A quadrilateral has interior angles 79°,101°,86° and d°. What is d?
What you will learn
- Explain the 360° quadrilateral sum using two triangles in a convex diagram.
- Solve and check equations formed from four labelled interior angles.
- Use the reflex interior at a concave vertex and distinguish it from the smaller outside sweep.
Draw a convex quadrilateral ABCD in boundary order, with no special-family marks. Mark its interior arcs A=88°,B=96°,C=109° and ∠CDA unknown. Draw internal diagonal AC to explain the angle total, then calculate ∠CDA. The drawing is not a licence to measure the missing angle.
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.
Quadrilateral ABCD
- α1 · arc-1: given 88°
- α2 · arc-2: given 96°
- α3 · arc-3: given 109°
- α4 · arc-4: given ?
Connect four interiors to two triangles
A quadrilateral is a four-sided polygon. For a convex quadrilateral ABCD, label the vertices in boundary order and draw the internal diagonal AC. It creates triangles ABC and ACD. Their combined 2×180° angle total is 360°, and the two parts at A and C rejoin the original quadrilateral interiors.
Use A+B+C+D=360° for the four interior angles, whether or not the shape is a special quadrilateral. The middle letter names an angle’s vertex: ∠CDA is at D, between DC and DA. Put each arc at its named vertex. Parallel arrows or equal-side marks are extra information and should not be added to an ordinary quadrilateral without a reason.
Turn the angle labels into an equation
If an angle is labelled with an expression, add the whole expression once. Brackets can help keep its constant term attached to the correct vertex. Collect the variable terms and constants, solve the equation and then substitute into every label. The variable itself is not usually the requested angle.
Check more than the algebraic manipulation. The four substituted interiors must total 360° and must be appropriate for the drawn figure. A convex quadrilateral has four interiors strictly between 0° and 180°. A total of 360° is necessary, but it would not on its own establish a promised parallel pair or a right angle.
Select the interior at an indentation
A simple concave quadrilateral still has total interior angle 360°. At its inward vertex, the interior follows the large reflex sweep inside the polygon rather than the smaller gap outside. Trace the polygon’s interior region and mark that long arc explicitly before adding angles.
The two sweeps between the same rays add to 360°. If the interior is 210°, the other sweep is 150°; they are different regions with different labels. Do not use a smaller outside angle in the quadrilateral sum. In an explanatory triangulation of a concave shape, choose a diagonal that really lies inside rather than extending the convenient convex construction without checking it.
Sources for this lesson
Pause and explain
A simple concave quadrilateral has interiors 75°,35°,40° and a reflex angle r. Which pair gives r and the other sweep at that vertex?
Worked example
Draw a convex quadrilateral ABCD in boundary order, with no special-family marks. Mark its interior arcs A=88°,B=96°,C=109° and ∠CDA unknown. Draw internal diagonal AC to explain the angle total, then calculate ∠CDA. The drawing is not a licence to measure the missing angle.
Show the worked solution
- The internal diagonal AC divides this convex figure into two triangles, so its four interior angles sum 2×180°=360°.
- The unknown is at D: ∠CDA=360°−88°−96°−109°=67°. Use the named vertex rather than transferring an arc to C.
- Check the full result: 88+96+109+67=360. Annotate 67° at D; no parallel or equal-side property was needed.
Answer ∠CDA=67°, because the two-triangle total is 360° and the three known interiors total 293°. The complete check is 88+96+109+67=360°.
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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 different convex quadrilateral ABCD in boundary order. Place A=(2x+14)°,B=(3x−6)°,C=(4x+2)°,D=80° on the four interior arcs. Find x and all four angles, and annotate a substituted total check. Keep the labels attached to their four named vertices and use a geometric reason before collecting the algebraic terms.
Give me a hint
Add all four interior labels, keeping each expression’s constant term.
Compare my reasoning
- Add the four labelled interiors: (2x+14)+(3x−6)+(4x+2)+80=360. Collecting terms gives 9x+90=360.
- Subtract 90 and divide by 9: 9x=270, so x=30. This is the variable value, before converting each label into an angle.
- Substitute at the named vertices: A=74°,B=84°,C=122°,D=80°. Each is between 0° and 180°, as required by the convex drawing.
- Check 74+84+122+80=360 and leave those exact degree labels on their arcs.
x=30; A=74°,B=84°,C=122°,D=80°. These four substituted convex interiors total 360°. Each result comes from replacing x in its own vertex expression, not treating x itself as one of the angles.
Look for these in your work
- Draw the named boundary and place every interior expression correctly.
- Solve 9x+90=360 and calculate all four angles.
- Check both the 360° sum and the convex angle ranges.
Draw and label convex quadrilateral ABCD with interiors A=(x+20)°,B=(2x+10)°,C=(3x−5)°,D=(4x+15)°. Form the angle equation, find x and calculate every interior angle. Keep each expression at the named vertex. Show which geometric total justifies the equation, and include degree units on every final interior label.
Give me a hint
Use the 360° sum first, then replace x in all four labels and check the total.
Compare my reasoning
- Add all four interior expressions once, because these are the interiors of one quadrilateral: (x+20)+(2x+10)+(3x−5)+(4x+15)=360. Collecting the terms gives 10x+40=360.
- Solve 10x=320, so x=32. Substitute into every expression rather than reporting 32° as an interior angle.
- Substitution gives A=52°,B=74°,C=91°,D=143°. Each lies strictly between 0° and 180°, which agrees with the stated convex boundary.
- Their sum is 52+74+91+143=360°, which checks the equation and the placement of all four labels.
x=32; A=52°,B=74°,C=91°,D=143°. The substituted angles total 360°, with the variable distinguished from every angle label.
Look for these in your work
- Form one complete four-interior equation.
- Find x=32 and substitute into each vertex label.
- Show a 360° check with all four computed degree values.
Draw a simple concave quadrilateral ABCD with C indented, A=90°,B=30°,D=30° and the long reflex interior at C labelled (5t+10)°. Mark that long interior arc; keep the other gap outside the polygon separately visible. Find t, the interior ∠BCD and the other sweep between CB and CD.
Give me a hint
Identify which sweep is inside the concave boundary before forming the angle equation.
Compare my reasoning
- At C the long interior arc, not the smaller outside gap, belongs in the quadrilateral sum. Write 90+30+(5t+10)+30=360.
- This simplifies to 5t+160=360, so t=40 and the reflex interior at C is 5(40)+10=210°.
- The other sweep between CB and CD is 360−210=150°. Label it outside the polygon rather than calling it the interior.
- Check the interiors: 90+30+210+30=360°. The reflex value is consistent with the stated concavity.
t=40; the interior ∠BCD is 210° and the other sweep is 150°. Only the 210° reflex interior belongs in the quadrilateral total.
Look for these in your work
- Draw a clear concave boundary and mark C’s long interior arc.
- Use that reflex expression in the 360° equation.
- Distinguish the 210° interior and 150° outside sweep with reasons.
Common mistakes
- Subtracting from 180° for a whole quadrilateral or adding an exterior gap as if it were interior.
- Solving for the variable but failing to calculate the four requested angle labels.
- Treating a 360° total alone as proof that an advertised special quadrilateral is possible.
What can you explain now?
In a simple concave quadrilateral ABCD, the interiors at A,B,D are 40°,50°,40°. The smaller outside gap between CB and CD is 130°. Which angle at C belongs in the quadrilateral sum?
Compare with the explanation
The reflex interior at C is 230°; use 230° in the sum.
The full-turn partner of the outside 130° is 360−130=230°. Then 40+50+230+40=360°. The smaller gap is outside, so it is not the requested interior.
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Make it stick
Explain why a quadrilateral has interior total 360°. Describe the checks needed after solving an angle equation, including how a reflex interior is chosen in a concave diagram.
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.