Two straight lines are parallel if they never intersect. |

The yellow lines are parallel |
The green lines are perpendicular |

The rectangle below has a perimeter measuring 3 + 4 + 3 + 4 = 14 cm, and an area of 12 cm².

A triangle which has 3 edges of the same length is called an equilateral triangle.

A triangle which has 2 edges of the same length is called an isosceles triangle.

A triangle which has a right angle is called a right-angled triangle.

The height of a triangle (red line) is the straight line which passes through a vertex and which is perpendicular to the opposite edge, called the base (green line).

The area of a triangle is equal to the length of the base (in green) times the length of the height (in red), divided by 2.

The triangle above is an equilateral triangle. |
The triangle above is an isosceles triangle. |
The triangle above is a right-angled triangle. |

The area of each of the three triangles above is equal to the length of the green edge times the length of the red segment divided by two.

The diagonals in a quadrilateral are the straight lines which join opposite vertices.

A quadrilateral whose opposite edges are parallel is called a

A quadrilateral which has 4 right angles is called a

A quadrilateral which has 4 right angles and 4 edges of the same length is called a

If the diagonals are perpendicular but do not have the same length, then the quadrilateral is a

The perimeter of a circle of radius r is equal to .

The area of a circle of radius r is equal to .