Theorems

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Theorems

Vertically opposite angles are equal in size.

In an isosceles triangle two sides and the two angles opposite these sides are equal.

Alternate angles are equal if two lines are parallel.

The angles in any triangle add up to 180'.

Two lines are parallel if the corresponding are equal.

Each exterior angle of a triangle is equal to the sum of the two interior opposite angles.

In a parallelogram , opposite sides and opposite angles are equal.

The diagonals of a parallelogram bisect each other.

If two triangles then their sides are proportional in order.

Pythagoras theorem: in a right-angled triangle, the square of the hypotenuse is equal is equalled to the sum of the squares of the two other sides.

If three parallel lines cut off equal segments on some transversal line, then they will cut off equal segments on any other transversal.

Let ABC be a triangle. If line l is parallel to BC and cuts [AB] in a ratio x:y, then it cuts [AC] in the same ratio.

The angle at the centre of a circle standing on a given arc is twice the angle at any point of the circle on the same arc.