Text 2. Basic geometric concepts

Date: 2015-10-07; view: 582.

The practical value of Geometry lies in the fact that we can abstract and illustrate physical objects by drawings and models. For example, a drawing of a circle is not a circle; it suggests the idea of a circle. In our study of Geometry we separate all geometric figures into two groups: plane figures whose points lie in one plane and space figures or solids. A point is a primary and starting concept in Geometry. Line segments, rays, triangles and circles are definite sets of points. A simple closed curve with line segments as its boundaries is a polygon. The line segments are sides of the polygon and the end points of the segments are vertices of the polygon. A polygon with four sides is a quadrilateral. We can name some important quadrilaterals. Remember, that in each case we name a specific set of points. A trapezoid is a quadrilateral with one pair of parallel sides. A rectangle is a parallelogram with four right angles. A square is a rectangle with all sides of the same length. Plane geometry is the science of the fundamental properties of the sizes and shapes of objects and treats geometric properties of figures. The first question is: Under what conditions two objects are equal (or congruent) in size and shape? Next, if figures are not equal, what significant relationship may they possess to each other and what geometric properties can they have in common? The basic relationship is shape. Figures of unequal size but of the same shape, that is, similar figures have many geometric properties in common. If figures have neither shape nor size in common, they may have the same area, or, in geometric terms, they may be equivalent, or may have endless other possible relationships. Geometry is the science of the properties, measurement and construction of lines, planes, surfaces and different geometric figures. How does a person find the area of a floor? Does he take little squares one foot on a side, lay them out over the entire floor and thus decide that the area of a floor is square feet, for this is indeed the meaning of area? Of course, he does not. He measures the length and width, quantities usually quite simple and then multiplies the two numbers to obtain the area. This is indirect measurement, for we find the area when we measure lengths. The dimensions we take in the case of volume are the area and the length or the height. Greek mathematicians are the founders of indirect measurement methods. Their contribution to this subject are formulae for areas and volumes of particular geometric shapes, that we use nowadays. Thus thanks to the Greeks we can find the area of any one single triangle when we take the product of its base and half its height. We also know due to them, that the «areas of two similar triangles are to each other as the squares of corresponding sides». In other words, even the very common formulae of Geometry which we owe to the Greeks permit us to measure areas and volumes indirectly, when we express these quantities as lengths. We ought not to undervalue this contribution of the ancient Greek mathematicians. Their formulae for areas and volumes represent a great practical and important result. But I this type of indirect measurement is not the only one of interest to the Greeks. They measure indirectly the radius of the Earth, the diameter of the sun and moon, the distances to the moon, the sun, some planets and stars.