U2: Angles in Geometric Figures

Key Learning Points:

The main properties are:

Simple straight lines

1. Sum of angles on a straight line is 180 deg.

2. Vertically Opposite Angles are equal (formed by 2 straight lines crossing each other like a big X).

3. Angles at a point adds up to 360 deg.

Triangles

4. Angles inside a triangle adds up to 180 deg.

5. Exterior angle is equal to the sum of the internal opposite angles.

6. Angles in an equilateral triangle are equal and each angle is 60 deg.

Parallelograms & Rhombuses

7. Internal opposite angles are equal.

8. Sum of the 2 angles bounded between 2 parallel lines adds up to 180 deg.

9. Sum of all angles inside a 4-sided figure (with at least a pair of parallel lines) is 360 deg.

Tips on solving questions:

• Highlight (on the diagram) the angles which you are required to find.
• Look out for the little square which indicates a 90 deg. Write down 90 deg above the square.
• Highlight all the straight lines (think about / spot rules 1 and 2).
• Highlight all the little straight lines on the triangles that indicates an isosceles or equilateral triangles (think about rules 5 and 6).
• Highlight all the arrows that indicate parallel lines (think about rules 7, 8 and 9).
• When dealing with rhombuses, think in terms of combining 2 isosceles triangles together.