Current Affairs 4th Class

Category : 4th Class

Geometry

Synopsis

Closed plane figures:

• The geometrical figures triangle, square, rectangle and circle are all closed plane figures or closed curves. They are bounded by line segments or curved lines.
• Closed plane figures, bounded by line segments are called polygons.
• A triangle is a polygon with 3 sides.
• Square and rectangle are polygons with four sides.
• All four sided polygons are named as quadrilaterals.
• Square, rectangle, parallelogram, rhombus and trapezium are all quadrilaterals.

                 (a)                (b)

            (c)           (d)

            (e)           (f)

Circle:

• A circle is a closed geometrical figure formed by points equidistant from a fixed point.

Circle

• Every circle has a fixed point in it called the centre of the circle, from which all the points on the circle are at the same distance.
• A line segment drawn from the centre of the circle to any point on the circle is called the radius of the circle. We can draw many radii (plural of radius) in a circle.

• A chord is a line segment whose end points always lie on the circle.
• A diameter is a chord that passes through the centre of the circle. We can draw many diameters to a circle. All the diameters of a circle are of the same length.

• The diameter is the biggest chord of a circle.
• Circumference of a circle is the total length of the boundary of the circle.
• Any part of a circle is called an arc.

• If an arc represents half of the circle, it is a semicircle or half-circle.
• A line which divides the given figure exactly into two identical figures is called the line of symmetry or axis of symmetry.

• A reflection is like a mirror image.

• Some shapes of tiles can fit in such a way that there are no gaps and overlaps. This tiling is called tessellation.