What is a proof?

Guest Apr 8, 2017




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

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