Coordinates. Graphical representation of functions
Coordinates. Coordinate system. Cartesian coordinates.
Axes of coordinates. Origin of coordinates. Abscissa and
ordinate. Graphical representation of functions. Graph
of a functional dependence.
Coordinates. Two mutually perpendicular straight lines XX’ and YY’ ( Fig.1 ) form
a coordinate system, called Cartesian coordinates. Straight lines XX’ and YY’ are called axes of
coordinates. The axis XX’ is called an xaxis, the axis YY’ – an yaxis. The point O of their intersection is
called an origin of coordinates. An arbitrary scale is selected on each axis of coordinates.
Find projections P and Q of a point M to the coordinate axes XX’
and YY’ . The segment OP on the axis XX’ and a number x, measuring its length according to the selected scale,
is called an abscissa or xcoordinate of a point M ; the segment OQ on axis YY’ and a
number y , measuring its length  an ordinate or ycoordinate of a point M.
Values x = OP and y = OQ are called Cartesian coordinates ( or simply – coordinates ) of a point M.
They are considered as positive or negative according to the adopted positive and negative directions of coordinate axes. Usually positive abscissas
are placed by right on an axis XX’ ; positive ordinates – by upwards on an axis YY’. On Fig.1 we
see: a point M has an abscissa x = 2, an ordinate y = 3; a point K has an abscissa x =
– 4 , an ordinate y = – 2.5. This can be written as: M ( 2, 3 ), K ( – 4, – 2.5 ). So,
for each point on a plane a pair of numbers (x, y) corresponds, and inversely, for each pair of real numbers (x, y) the
one point on a plane corresponds .
Graphical representation of functions.
To represent a functional dependence y = f ( x ) as a graph it is necessary:
1) to write a set of values of the function and its argument in a table:
2) To transfer the coordinates of the function points from the table to a coordinate system,
marking according to the selected scale a set of xcoordinates on xaxis and a set of
ycoordinates on yaxis ( Fig.2 ). As a result a set of points
A, B, C, . . . , F will be
plotted in our coordinate system.
3) Joining marked points A, B, C, . . . , F by a smooth curve, we receive a graph of the given
functional dependence.
Such graphical representation of a function permits to visualize a behavior of the function, but has an insufficient attainable
accuracy. It’s possible, that intermediate points, not plotted on a graph, lie far from the drawing smooth curve. Good results also depend essentially
on a successful choice of scales. That is why, you should define a graph of a function as a locus, coordinates
of points of which M (x, y) are tied by the given functional dependence.
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