+1886 Answer
| Reason | Abbreviation |
| Vertically opposite angles are equal | vert opp Ðs |
| Adjacent angles on a straight line add to 180º | Adj, Ðs on str. line |
| Angles at a point add to 360º | Ðs at pt |
| Angles in a triangle add to 180º | Ð sum of Δ |
| The exterior angle of a triangle equals the sum of the two interior opposite angles | ext Ð of Δ |
| The base angles of an isosceles triangle are equal | base Ðs isos. Δ |
| The angle sum of an isosceles triangle is 180º | Ð sum isos. Δ |
| Each angle in an equilateral triangle is 60º | Ð in equilat. Δ |
| Corresponding angles on parallel lines are equal | corresp Ðs, // lines |
| Alternate angles on parallel lines are equal | alt. Ðs, // lines |
| Co-interior angles on parallel lines are supplementary (add to 180º) | co-int Ðs, // lines |
| The interior angles of a polygon add to 180(n – 2)º, where n is the number of sides | int Ð sum of polygon |
| The exterior angles of a polygon add to 360 | ext Ð sum of polygon |
| Isosceles triangle, equal radii | isos Δ, = radii |
| Angles of isosceles triangle add to 180º , equal radii | Ð sum isos Δ, = radii |
| Base angles of isosceles triangle are equal, equal radii | base Ðs, isos Δ, = radii |
| The angle at the centre is equal to twice the angle at the circumference on the same arc | Ð at centre |
| Angles on the same arc are equal | Ðs on same arc |
| The angle in a semicircle is a right angle | Ð in semicircle |
| Opposite angle of a cyclic quadrilateral are supplementary (add to 180º) | opp Ðs, cyclic quad |
| The exterior angle of a cyclic quadrilateral is equal to the interior opposite angle | ext Ð, cyclic quad |
| The perpendicular from the centre to the chord bisects the chord | from centre to chord |
| The angle where the radius meets the tangent is 90º | tgt rad |
| Tangents from a point to a circle are the same length | equal tangents |
