- Rio de Janerio, Brazil
- The Hague, The Netherlands
Hindu–Arabic numeral system
The Hindu–Arabic numeral system, sometimes known as Hindu–Arabic numbers, is the system of numbers used by almost all the world today. It is a positional notation system. This system has ten basic symbols, they are 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9. It represent numbers in the decimal number system. The Hindu–Arabic numeral system was first developed by the Hindus. Later, It was introduced to the western world by the Arabs. The Hindu–Arabic numeral system is one of the few numeral systems that use a place value system. This system is now commonly used all over the world.
They originated in India in the 6th or 7th century and were introduced to Europe through the writings of Middle Eastern mathematicians, especially al-Khwarizmi and al-Kindi, about the 12th century. They are very different from previous methods of counting, such as the abacus, and paved the way for the development of algebra. In the past many other systems were used. Fibonacci popularized the Hindu–Arabic numeral system to the Western World.
Point (geometry)
A point is a precise position in space. Imagine touching a piece of paper with a sharp pencil or pen, without making any sideways movement. We know where the point is, but it has no size to speak of.
In geometry, a point has no size, but has a position. This means it has no volume, area or length. We usually represent a point by a small cross 'X' or by a small dot (a small, round shape). Points are always labelled by capital letters (A, B, C...X, Y, Z). It is one of the most fundamental but undefined terms in geometry.
In general, two points can be:
and are always:
Three points can be:
and are always:
Four points can be:
Line
A line is the path of one point moving. A line has length but no width. A line is a type of geometric figure.
A line is made up of an endless number of points.
A line can be straight or curved. In geometry, the word "line" means a "straight line". A straight line is the shortest distance between two points. A straight line is the line traced by a point moving in a direction that does not change.
A "curved line" is sometimes called a curve. The edge of a circle is not straight and is an example of a curve.
A "line segment" is part of a line. Here is an example of a straight line segment:
Lines can get their names from any two points on the line. For example, if a line contains two distinct points, named formula_1 and formula_2, then the line can be called either formula_3, or formula_4.
Sometimes, lines are also named with just one symbol. In which case, a letter such as formula_5 might be used.
Two lines can be:
Plane (mathematics)
A plane is a perfectly flat surface extending in all directions. It can be thought of as the ceiling of a room, only extended into all directions infinitely. A plane has two dimensions: length and width. All planes are flat surfaces. If a surface is not flat, it is called a "curved surface".
The tool "plane" can be used to create a flat, level surface like the mathematical plane—hence the name.
In geometry, a plane is made up of an infinite number of lines (or points). It has no depth. It is absolutely flat and infinitely large.
A "plane figure" is part of a plane. It is named by the capital letters (such as A, B, C, ...X, Y, Z) that are put at its corners. Sometimes, a single capital pi formula_1 is also used to refer to a plane. A plane can also be named after three points that are not all on the same line.
In general, two planes can be:
In mathematics, a plane is a fundamental two-dimensional object. Intuitively, it looks like a flat infinite sheet of paper. There are several definitions of the plane. They are equivalent in the sense of Euclidean geometry, but they can be extended in different ways to define objects in other areas of mathematics. The only two-dimensional figure in our three-dimensional world is a shadow.
In some areas of mathematics, such as plane geometry or 2D computer graphics, the whole space in which the work is carried out is a single plane. In such situations, the definite article is used: "the plane". Many fundamental tasks in geometry, trigonometry and graphing are performed in the two dimensional space, or in other words, in the plane.
A plane is a surface such that, given any three distinct points on the surface, the surface also contains all of the straight lines that pass through any two of them.
One can introduce a Cartesian coordinate system on a given plane in order to label every point on it with a unique ordered pair, which is composed of two numbers and is the coordinate of the point.
Within any Euclidean space, a plane is uniquely determined by any of the following combinations:
Angle
When two straight lines come together, they make an angle. The two lines are called the "sides" of the angle, and they meet at a point. A flat surface (called a plane) also forms an angle when it meets another.
To represent an angle, Greek letters such as formula_1 (alpha), formula_2 (beta), formula_3 (gamma) and formula_4 (theta) are sometimes used. An angle indicates the space between its sides, or the amount of rotation needed to make one side coincide the other.
To measure the size of an angle, we use units called degrees. A degree is a standard unit and we use the symbol ° after a number to show that it is a number of degrees. We can use a decimal number or a fraction for part of a degree, but a degree can also be divided into 60 minutes (1° = 60'), and a minute can be divided into 60 seconds (1' = 60"). So 22.5°, 22° and 22° 30' are all the same angle.
In mathematics, angles can also be (and often are) measured in radians instead of degrees, by using the conversion factor formula_5 (for example, formula_6). Yet another unit of angle is gradian, with formula_7.
Angles are studied in geometry, where an angle where edges meet is often called a vertex. For example, the three sides of a triangle are its "edges" and two of the edges meet at each "vertex". Similarly, two of the six sides (or "faces") of a cube meet at each of its twelve edges, and three edges meet at each of its eight corners (or vertices, which is the plural version of "vertex").
•In a zero angle the lines lie one upon the other thus creating a 0° angle aka the zero angle.
•An angle greater than 0° but less than 90° is called an acute angle.
•An angle of 90° is called a right angle.
An angle greater than 90° but less than 180° is called an obtuse angle.
•An angle that measures 180° is called a straight angle.
An angle greater than 180° and less than 360° is called a reflex angle.
•An angle that has a made or full circle/ completed 360° is called a full or complete angle.
Supplementary angles are two angles with the sum equal to 180°, and complementary angles are two angles with the sum equal to one right angle (90°). On the other hand, two angles that sum to one full circle (360°) are sometimes called explementary angles, or conjugate angles.
People usually use a protractor to measure and draw angles. Sometimes, people use a 360° protractor to measure angles.
List of astronomers
Famous astronomers include:
Other scientists whose work had an impact on astronomy and astrophysics:
Instrument makers, illustrators, etc.:
List of astrophysicists
An astrophysicist is a person whose profession is astrophysics.
List of walls
This is a list of famous walls.
List of vegetables
Some vegetables which are botanically fruits (such as tomatoes) are considered to be vegetables in the culinary (eating) sense. This is why they appear in this article. For sources, see individual pages.
List of terrorist incidents
The following is a list by date of acts and failed attempts that can be considered terrorism.
Note: there is no single official definition of terrorism in common use. Events listed here are commonly called terrorism, or meet some of the commonly used parts of the definition.