Geometry

**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.**

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