Triangle Angle Theorems — O/L Mathematics

What must I remember about Triangle Angle Theorems?

• Angles on a straight line add up to 180°; angles around a single point add up to 360°.
• Vertically opposite angles formed by two intersecting straight lines are equal.
• Parallel lines cut by a transversal produce three sets of equal/supplementary angle relationships: Alternate angles are equal (Z-shape).
• Corresponding angles are equal (F-shape).
• Allied (co-interior) angles are supplementary, summing to 180° (C-shape or U-shape).

Test yourself (1 minute)

1. Parallel lines requirement

   Alternate and corresponding angles are ONLY equal when the lines are strictly parallel; always confirm parallel markings (arrows) before equating.

2. Angle theorem abbreviation standards

   In Paper 1 Part A, write recognized reasons: 'alt. ∠s, AB // CD', 'corr. ∠s, PQ // RS', 'vert. opp. ∠s'.

3. Constructing auxiliary parallel lines

   In stepped zig-zag lines between parallel boundaries, draw an auxiliary parallel line through the vertex to decompose the angle.

Practise Triangle Angle Theorems questions today

Source: NIE G.C.E. (O/L) Mathematics Syllabus (Grade 10, Unit 23)

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Grade 10 · Term 3