How do I tell corresponding, alternate and co-interior angles apart?

Look at the two angles relative to the transversal: same side and same position gives corresponding angles, opposite sides inside the parallels gives alternate angles, same side inside gives co-interior angles.

Must the degree symbol be written in the answer?

Yes. Always give the unit with the size of an angle or marks are lost for the final answer.

S1 Maths: Angles and Parallel Lines, Paper 1 (Basic)

2026-11-09

Tutor Prep Guide · S1 Maths

Ten basic items on adjacent angles on a straight line, vertically opposite angles, complements, corresponding, alternate and co-interior angles, each with an answer and a one-line explanation.

How are the angles formed by parallel lines related?　When a transversal cuts two parallel lines: corresponding angles are equal, alternate angles are equal, and co-interior angles add up to 180 degrees.

01　Paper details (check answers afterwards)

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

02　Questions (10 items)

FoundationOn a straight line, ∠AOB = 65° and ∠BOC is adjacent to it. Find ∠BOC.
FoundationTwo straight lines intersect. One of the four angles is 48°. Find its vertically opposite angle.
FoundationFind the complement of 37°.
IntermediateA transversal cuts two parallel lines. One pair of co-interior angles has one angle of 108°. Find the other.
IntermediateA transversal cuts two parallel lines. A pair of alternate angles is 3x and x + 40. Find x and the size of the angle.
IntermediateTwo parallel lines are cut by a transversal. A pair of co-interior angles is 2x and 3x. Find x and the two angles.
ChallengeThree angles meet at a point. Two of them are 120° and 95°. Find the third angle.
ChallengeA transversal cuts two parallel lines. A pair of co-interior angles is in the ratio 2 : 3. Find the two angles and the supplement of the smaller one.
ChallengeThree angles x, 2x and 3x lie on a straight line. Find x and the three angles.
Error redoA student claims co-interior angles are equal. Given one co-interior angle of 115°, find the other and give the correct reason.

03　Answers and one-line notes

#	Answer	One-line note
1	∠BOC = 115°	Adjacent angles on a straight line add up to 180°.
2	The angle is 48°	Vertically opposite angles are equal.
3	The complement is 53°	Complementary angles add up to 90°.
4	The other co-interior angle is 72°	Co-interior angles between parallel lines add up to 180°.
5	x = 20, the angle is 60°	Alternate angles are equal, so 3x = x + 40.
6	x = 36, the angles are 72° and 108°	Co-interior angles add up to 180, so 2x + 3x = 180.
7	The third angle is 145°	Angles at a point add up to 360°.
8	The angles are 72° and 108°; the supplement of the smaller one is 108°	Split 180° in the ratio 2 : 3, then take the supplement.
9	x = 30, the angles are 30°, 60° and 90°	Adjacent angles on a straight line add up to 180, so x + 2x + 3x = 180.
10	The other is 65°; co-interior angles are supplementary, not equal	Co-interior angles add up to 180; it is the alternate and corresponding angles that are equal.

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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Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.