G1. Angles, triangles and polygons (Mathematical Properties & Formulae-S1)

Right angle

= 90°

Acute angle

< 90°

Obtuse angle

> 90° and < 180°

Reflex angle

> 180° and < 360°

Vertically opposite angles are equal

Hai góc đối đỉnh bằng nhau

Angles on a straight line sum to 180°

Các góc trên một đường thẳng có tổng bằng 180°

Angles at a point sum to 360°

Các góc tại một điểm có tổng bằng 360°

Interior angles

Tổng bằng 180°

Sum of interior angles

= 180°

Equilateral

all angles = 60°

Isosceles

2 equal angles

Right-angled triangle

Có một góc vuông

Square

- 4 equal sides, 4 right angles, all sides parallel in pairs
- Diagonals are equal, bisect each other and perpendicular
- Diagonals bisects the square into congruent isosceles right triangles.
- Diagonals are the bisectors of angles.

Rectangle

- Opposite sides are equal and parallel, 4 right angles
- Diagonals are equal and bisect each other

Rhombus

- 4 equal sides, opposite angles are equal.
- Diagonals bisect each other at right angles.
- The diagonal divides the figure into two equilateral triangles.
- Diagonals are the bisectors of angles.

Parallelogram

- Opposite sides are equal and parallel, opposite angles are equal.
- Diagonals bisect each other.
- Diagonals divide each other into two equal halves.

Trapezium

- Only one pair of opposite sides are parallel.
- Adjacent angles between parallel lines are supplementary.

Kite

- Two pairs of adjacent sides are equal.
- One pair of opposite angles are equal, diagonals are perpendicular.

Interior angle sum

Sum = (n − 2) × 180°

Each interior angle of regular polygon

(n − 2) × 180°/ n

Each exterior angle

360°/ n