Parallel lines, perpendicular lines
Two straight lines are parallel if they never intersect.Two straight lines are perpendicular if they form a right angle, that is, an angle that measures 90 degrees.
The yellow lines are parallel |
The green lines are perpendicular |
Perimeter, area
The perimeter of a figure is the length of the circumference of the figure, the area of a figure is its surface, that is the number of small squares of edge 1 that we can put inside.
Triangles
A triangle is a figure which has 3 edges. There are several types of triangles :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.
Quadrilaterals
A quadrilateral is a figure which possesses 4 edges.The diagonals in a quadrilateral are the straight lines which join opposite vertices.
A quadrilateral whose opposite edges are parallel is called a parallelogram. Its diagonals intersect at their centre. Conversely, if the diagonals of a quadrilateral intersect at their centre, then that quadrilateral is a parallelogram.
Parallelogram :
A quadrilateral which has 4 right angles is called a rectangle. Its diagonals intersect at their centre and have same length.
Rectangle :
A quadrilateral which has 4 right angles and 4 edges of the same length is called a square. Its diagonals intersect at their centre, have same length, and are perpendicular.
Square :
If the diagonals are perpendicular but do not have the same length, then the quadrilateral is a diamond. It is a quadrilateral which has 4 edges of the same length.
Diamond :
Circle
The number
is approximately 3,14. The perimeter of a circle of radius r is equal to
.The area of a circle of radius r is equal to
.
See also : geometry on fmaths.com