A Cartesian coordinate system specifies each point uniquely in a plane by a pair of numerical coordinates, which are the signed distances from the point to two fixed perpendicular directed lines, measured in the same unit of length.
Each reference line is called a coordinate axis or just axis of the system, and the point where they meet is its origin. The coordinates can also be defined as the positions of the perpendicular projections of the point onto the two axes, expressed as a signed distances from the origin.
One can use the same principle to specify the position of any point in threedimensional space by three Cartesian coordinates, its signed distances to three mutually perpendicular planes (or, equivalently, by its perpendicular projection onto three mutually perpendicular lines). In general, one can specify a point in a space of any dimension n by use of n Cartesian coordinates, the signed distances from n mutually perpendicular hyperplanes.
The invention of Cartesian coordinates in the 17th century by René Descartes revolutionized mathematics by providing the first systematic link between Euclidean geometry and algebra. Using the Cartesian coordinate system, geometric shapes (such as curves) can be described by Cartesian equations: algebraic equations involving the coordinates of the points lying on the shape. For example, the circle of radius 2 may be described as the set of all points whose coordinates x and y satisfy the equation x^{2} + y^{2} = 2^{2}.
Cartesian coordinates are the foundation of analytic geometry, and provide enlightening geometric interpretations for many other branches of mathematics, such as linear algebra, complex analysis, differential geometry, multivariate calculus, group theory, and more. A familiar example is the concept of the graph of a function. Cartesian coordinates are also essential tools for most applied disciplines that deal with geometry, including astronomy, physics, engineering, and many more. They are the most common coordinate system used in computer graphics, computeraided geometric design, and other geometryrelated data processing.
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The adjective Cartesian refers to the French mathematician and philosopher René Descartes (who used the name Cartesius in Latin).
The idea of this system was developed in 1637 in two writings by Descartes and independently by Pierre de Fermat, although Fermat used 3 dimensions and did not publish the discovery.^{[1]} In part two of his Discourse on Method, Descartes introduces the new idea of specifying the position of a point or object on a surface, using two intersecting axes as measuring guides.^{[citation needed]} In La Géométrie, he further explores the abovementioned concepts.^{[2]}
It may be interesting to note that some have indicated that the master artists of the Renaissance used a grid, in the form of a wire mesh, as a tool for breaking up the component parts of their subjects they painted. That this may have influenced Descartes is merely speculative.^{[citation needed]} (See perspective, projective geometry.) Representing a vector in the standard basis. The development of the Cartesian coordinate system enabled the development of calculus by Isaac Newton and Gottfried Wilhelm Leibniz.^{[3]}
Although Descartes lends his name to the system, scholars before him have used two and threepoint coordinate systems, including Abu Rayhan Biruni, a tenth and eleventh century Persian polymath.^{[4]}
Many other coordinate systems have been developed since Descartes, such as the polar coordinates for the plane, and the spherical and cylindrical coordinates for threedimensional space.
Choosing a Cartesian coordinate system for a onedimensional space—that is, for a straight line—means choosing a point O of the line (the origin), a unit of length, and an orientation for the line. The latter means choosing which of the two halflines determined by O is the positive, and which is negative; we then say that the line is oriented (or points) from the negative half towards the positive half. Then each point p of the line can be specified by its distance from O, taken with a + or − sign depending on which halfline contains p.
A line with a chosen Cartesian system is called a number line. Every real number, whether integer, rational, or irrational, has a unique location on the line. Conversely, every point on the line can be interpreted as a number in an ordered continuum which includes the real numbers.
Choosing a Cartesian coordinate system for a plane means choosing an ordered pair of lines (axes) at right angles to each other, a single unit of length for both axes, and an orientation for each axis. The point where the axes meet is taken as the origin for both axes, thus turning each axis into a number line. Each coordinate of a point p is obtained by drawing a line through p perpendicular to the associated axis, finding the point q where that line meets the axis, and interpreting q as a number of that number line.
Choosing a Cartesian coordinate system for a threedimensional space means choosing an ordered triplet of lines (axes), any two of them being perpendicular; a single unit of length for all three axes; and an orientation for each axis. As in the twodimensional case, each axis becomes a number line. The coordinates of a point p are obtained by drawing a line through p perpendicular to each coordinate axis, and reading the points where these lines meet the axes as three numbers of these number lines.
Alternatively, the coordinates of a point p can also be taken as the (signed) distances from p to the three planes defined by the three axes. If the axes are named x, y, and z, then the x coordinate is the distance from the plane defined by the y and z axes. The distance is to be taken with the + or − sign, depending on which of the two halfspaces separated by that plane contains p. The y and z coordinates can be obtained in the same way from the (x,z) and (x,y) planes, respectively.
One can generalize the concept of Cartesian coordinates to allow axes that are not perpendicular to each other, and/or different units along each axis. In that case, each coordinate is obtained by projecting the point onto one axis along a direction that is parallel to the other axis (or, in general, to the hyperplane defined by all the other axes). In those oblique coordinate systems the computations of distances and angles is more complicated than in standard Cartesian systems, and many standard formulas (such as the Pythagorean formula for the distance) do not hold.
The Cartesian coordinates of a point are usually written in parentheses and separated by commas, as in (10,5) or (3,5,7). The origin is often labelled with the capital letter O. In analytic geometry, unknown or generic coordinates are often denoted by the letters x and y on the plane, and x, y, and z in threedimensional space. w is often used for fourdimensional space, but the rarity of such usage precludes concrete convention here. This custom comes from an old convention of algebra, to use letters near the end of the alphabet for unknown values (such as were the coordinates of points in many geometric problems), and letters near the beginning for given quantities.
These conventional names are often used in other domains, such as physics and engineering. However, other letters may be used too. For example, in a graph showng how a pressure varies with time, the graph coordinates may be denoted t and P. Each axis is usually named after the coordinate which is measured along it; so one says the xaxis, the yaxis, the taxis, etc.
Another common convention for coordinate naming is to use subscripts, as in x_{1}, x_{2}, ... x_{n} for the n coordinates in an ndimensional space; especially when n is greater than 3, or variable. Some authors (and many programmers) prefer the numbering x_{0}, x_{1}, ... x_{n−1}. These notations are especially advantageous in computer programming: by storing the coordinates of a point as an array, instead of a record, one can use iterative commands or procedure parameters instead of repeating the same commands for each coordinate.
In mathematical illustrations of twodimensional Cartesian systems, the first coordinate (traditionally called the abscissa) is measured along a horizontal axis, oriented from left to right. The second coordinate (the ordinate) is then measured along a vertical axis, usually oriented from bottom to top.
However, in computer graphics and image processing one often uses a coordinate system with the y axis pointing down (as displayed on the computer's screen). This convention developed in the 1960s (or earlier) from the way that images were originally stored in display buffers.
For threedimensional systems, mathematicians usually draw the z axis as vertical and pointing up, so that the x and y axes lie on an horizontal plane. There is no prevalent convention for the directions of these two axes, but the orientations are usually chosen according to the righthand rule. In three dimensions, the names "abscissa" and "ordinate" are rarely used for x and y, respectively. When they are, the zcoordinate is sometimes called the applicate.
The axes of a twodimensional Cartesian system divide the plane into four infinite regions, called quadrants, each bounded by two halfaxes. These are often numbered from 1st to 4th and denoted by Roman numerals: I (where the signs of the two coordinates are (+,+), II (−,+), III (−,−), and IV (+,−). When the axes are drawn according to the mathematical custom, the numbering goes counterclockwise starting from the upper right ("northeast") quadrant.
Similarly, a threedimensional Cartesian system defines a division of space into eight regions or octants, according to the signs of the coordinates of the points. The octant where all three coordinates are positive is sometimes called the first octant; however, there is no established nomenclature for the other octants. The ndimensional generalization of the quadrant and octant is the orthant.
A Euclidean plane with a chosen Cartesian system is called a Cartesian plane. Since Cartesian coordinates are unique and nonambiguous, the points of a Cartesian plane can be identified with all possible pairs of real numbers; that is with the Cartesian product , where is the set of all reals. In the same way one defines a Cartesian space of any dimension n, whose points can be identified with the tuples (lists) of n real numbers, that is, with .
The Euclidean distance between two points of the plane with Cartesian coordinates (x_{1},y_{1}) and (x_{2},y_{2}) is
This is the Cartesian version of Pythagoras' theorem. In threedimensional space, the distance between points (x_{1},y_{1},z_{1}) and (x_{2},y_{2},z_{2}) is
which can be obtained by two consecutive applications of Pythagoras' theorem.
Translating a set of points of the plane, preserving the distances and directions between them, is equivalent to adding a fixed pair or numbers (X,Y) to the Cartesian coordinates of every point in the set. That is, if the original coordinates of a point are (x,y), after the translation they will be
To make a figure larger or smaller is equivalent to multiplying the Cartesian coordinates of every point by the same positive number m. If (x,y) are the coordinates of a point on the original figure, the corresponding point on the scaled figure has coordinates
If m is greater than 1, the figure becomes larger; if m is between 0 and 1, it becomes smaller.
To rotate a figure counterclockwise around the origin by some angle θ is equivalent to replacing every point with coordinates (x,y) by the point with coordinates (x',y'), where
If (x, y) are the Cartesian coordinates of a point, then (−x, y) are the coordinates of its reflection across the second coordinate axis (the Y axis), as if that line were a mirror. Likewise, (x, −y) are the coordinates of its reflection across the first coordinate axis (the X axis).
The Euclidean transformations of the plane are the translations, rotations, scalings, reflections, and arbitrary compositions thereof. The result (x',y') of applying a Euclidean transformation to a point (x,y) is given by the formula
where A is a 2×2 matrix and b is a pair of numbers, that depend on the transformation; that is,
The matrix A must have orthogonal rows with same Euclidean length, that is,
and
This is equivalent to saying that A times its transpose must be a diagonal matrix. If these conditions do not hold, the formula describes a more general affine transformation of the plane.
The formulas define a translation if and only if A is the identity matrix. The transformation is a rotation around some point if and only if A is a rotation matrix, meaning that
Fixing or choosing the xaxis determines the yaxis up to direction. Namely, the yaxis is necessarily the perpendicular to the xaxis through the point marked 0 on the xaxis. But there is a choice of which of the two half lines on the perpendicular to designate as positive and which as negative. Each of these two choices determines a different orientation (also called handedness) of the Cartesian plane.
The usual way of orienting the axes, with the positive xaxis pointing right and the positive yaxis pointing up (and the xaxis being the "first" and the yaxis the "second" axis) is considered the positive or standard orientation, also called the righthanded orientation.
A commonly used mnemonic for defining the positive orientation is the right hand rule. Placing a somewhat closed right hand on the plane with the thumb pointing up, the fingers point from the xaxis to the yaxis, in a positively oriented coordinate system.
The other way of orienting the axes is following the left hand rule, placing the left hand on the plane with the thumb pointing up.
When pointing the thumb away from the origin along an axis, the curvature of the fingers indicates a positive rotation along that axis.
Regardless of the rule used to orient the axes, rotating the coordinate system will preserve the orientation. Switching any two axes will reverse the orientation.
Once the x and yaxes are specified, they determine the line along which the zaxis should lie, but there are two possible directions on this line. The two possible coordinate systems which result are called 'righthanded' and 'lefthanded'. The standard orientation, where the xyplane is horizontal and the zaxis points up (and the x and the yaxis form a positively oriented twodimensional coordinate system in the xyplane if observed from above the xyplane) is called righthanded or positive.
The name derives from the righthand rule. If the index finger of the right hand is pointed forward, the middle finger bent inward at a right angle to it, and the thumb placed at a right angle to both, the three fingers indicate the relative directions of the x, y, and zaxes in a righthanded system. The thumb indicates the xaxis, the index finger the yaxis and the middle finger the zaxis. Conversely, if the same is done with the left hand, a lefthanded system results.
Figure 7 depicts a left and a righthanded coordinate system. Because a threedimensional object is represented on the twodimensional screen, distortion and ambiguity result. The axis pointing downward (and to the right) is also meant to point towards the observer, whereas the "middle" axis is meant to point away from the observer. The red circle is parallel to the horizontal xyplane and indicates rotation from the xaxis to the yaxis (in both cases). Hence the red arrow passes in front of the zaxis.
Figure 8 is another attempt at depicting a righthanded coordinate system. Again, there is an ambiguity caused by projecting the threedimensional coordinate system into the plane. Many observers see Figure 8 as "flipping in and out" between a convex cube and a concave "corner". This corresponds to the two possible orientations of the coordinate system. Seeing the figure as convex gives a lefthanded coordinate system. Thus the "correct" way to view Figure 8 is to imagine the xaxis as pointing towards the observer and thus seeing a concave corner.
A point in space in a Cartesian coordinate system may also be represented by a vector, which can be thought of as an arrow pointing from the origin of the coordinate system to the point. If the coordinates represent spatial positions (displacements) it is common to represent the vector from the origin to the point of interest as . In three dimensions, the vector from the origin to the point with Cartesian coordinates (x,y,z) is sometimes written as^{[5]}:
where , , and are unit vectors and the respective versors of x, y, and z axes. This is the quaternion representation of the vector, and was introduced by Sir William Rowan Hamilton. The unit vectors , , and are called the versors of the coordinate system, and are the vectors of the standard basis in threedimensions.
Each axis may have different units of measurement associated with it (such as kilograms, seconds, pounds, etc.). Although four and higherdimensional spaces are difficult to visualize, the algebra of Cartesian coordinates can be extended relatively easily to four or more variables, so that certain calculations involving many variables can be done. (This sort of algebraic extension is what is used to define the geometry of higherdimensional spaces.) Conversely, it is often helpful to use the geometry of Cartesian coordinates in two or three dimensions to visualize algebraic relationships between two or three of many nonspatial variables.
The graph of a function or relation is the set of all points satisfying that function or relation. For a function of one variable, f, the set of all points (x,y) where y = f(x) is the graph of the function f. For a function of two variables, g, the set of all points (x,y,z) where z = g(x,y) is the graph of the function g. A sketch of the graph of such a function or relation would consist of all the salient parts of the function or relation which would include its relative extrema, its concavity and points of inflection, any points of discontinuity and its end behavior. All of these terms are more fully defined in calculus. Such graphs are useful in calculus to understand the nature and behavior of a function or relation.
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