How do I tell a supplement from a complement?

A supplement pairs with 180° and a complement with 90°: use the straight line for supplements and the right angle for complements.

Must the degree sign be written?

Yes. Always write the degree sign, or marks are lost.

S1 Maths: Angles and Parallel Lines, Paper 4 (Error redo)

2026-11-09

Tutor Prep Guide · Primary Maths

Ten redo items on the mistakes students actually make: mixing up supplement and complement, treating alternate angles as supplementary, and dropping the degree sign.

Where do parallel-line questions usually go wrong?　Three danger spots: confusing the supplement (180°) with the complement (90°), writing alternate angles as supplementary, and forgetting the degree sign. Check every answer against 180° or 90°.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S1 Maths: Angles and Parallel Lines
Level	Level: Error redo

02　Questions (10 items)

FoundationFind the supplement of 62°.
FoundationFind the complement of 28°.
FoundationTwo straight lines intersect. One of the four angles is 74°. Find its vertically opposite angle.
FoundationA transversal cuts two parallel lines. One co-interior angle is 115°. Find the other co-interior angle.
IntermediateA transversal cuts two parallel lines. A pair of alternate angles is 4x and x + 60. Find x and the angle.
IntermediateA transversal cuts two parallel lines. A pair of co-interior angles is 3x and 2x. Find x and both angles.
IntermediateA transversal cuts two parallel lines. A pair of corresponding angles is (2x + 10)° and (3x − 20)°. Find x and the angle.
ChallengeThree angles meet at a point. Two of them are 130° and 85°. Find the third angle.
ChallengeA transversal cuts two parallel lines. A pair of co-interior angles is in the ratio 3 : 7. Find both angles.
Error redoA student writes that vertically opposite angles are always supplementary. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	The supplement is 118°	Supplementary angles add up to 180°.
2	The complement is 62°	Complementary angles add up to 90°.
3	The angle is 74°	Vertically opposite angles are equal, so no subtraction is needed.
4	The other angle is 65°	Co-interior angles add up to 180°, so 180° − 115°.
5	x = 20, the angle is 80°	Alternate angles are equal: 4x = x + 60, so x = 20.
6	x = 36, the angles are 108° and 72°	Co-interior angles add up to 180: 3x + 2x = 180, so x = 36.
7	x = 30, the angle is 70°	Corresponding angles are equal: 2x + 10 = 3x − 20, so x = 30.
8	The third angle is 145°	Angles at a point add up to 360°.
9	The angles are 54° and 126°	Split 180° into ten parts of 18° in the ratio 3 : 7.
10	It confuses equal with supplementary: vertically opposite angles are equal, and only add to 180° when each is 90°.	Vertically opposite angles are equal; supplementary describes a sum of 180°, which is a different relation.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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