Types and Properties of Angles
Angles which measure exactly 180° (degrees) are straight angles. Therefore, straight angles are straight lines. Angles are represented by the sign ϴ, called theta. That is, for straight angles, ϴ= 180°.
Angles which measure exactly 90° are right angles, that is, ϴ = 90°.
Obtuse angles are those which are greater than 90° but less than 180°, that is, 90° < ϴ < 180°.
Acute angles are angles which are greater than 0° but less than 90°, that is, 0° < ϴ < 90°.
Reflex angles are angles which are greater than 180° but less than 360°, that is, 180° < ϴ < 360°.
Two angles which share the same vertex (centre, usually represented by 0) and have a common side (line) are called adjacent angles.
Complementary angles are two angles which when summed equals 90°.
Note: <A and <B, are ‘angle A’ and ‘angle B’ respectively.
Supplementary angles are two angles which when summed equals 180°.
Vertically Opposite Angles
Vertically opposite angles are the angles opposite to each other when two straight lines intersect. Their defining property is that, vertically opposite angles are equal in magnitude.
When two parallel lines are crossed by a line called the transversal, the angles formed which are in corresponding positions, are called corresponding angles. Corresponding angles are equal in magnitude.