Back to: Mathematics Primary 6
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A plane surface forms the plane figure of limited surfaces. We know about the following plane figures:
(i) Rectangle
(ii) Square
(iii) Triangle
(iv) Circle
These are all closed plane figures closed by line-segments. Now, we have to learn more about such closed figures.
Rectangle: A rectangle is a plane closed figure enclosed by four-line segments.
These four line segments are called the sides of the rectangle. In the above rectangle ABCD the sides are AB, DC, AD and BC.
Thus, there are four sides of a rectangle. The opposite sides are equal to each other. A rectangle has four vertices.
Square: A square is plane closed figure enclosed by four equal line-segments.
- It is also like a rectangle having all four sides equal. Like rectangle, the line-segments of the square are also known as sides. In the above square DEFG, there are four sides DE, EF, FG and GD; these sides are equal and they are four vertices D, E, F and G.
Thus, the closed figure of square has four equal sides and four vertices. A square is a rectangle having four equal sides.
Triangle: A triangle is a plane closed figure enclosed by three line-segments.
The line-segments become the sides of the triangle and enclose it.
There are triangles having three sides of different lengths. Every triangle has three vertices. In the above, triangle ABC, DEF and PQR are of different shapes.
Thus, a triangle has three sides and three vertices.
Circle: There is a plane closed figure shown here. It is different from that of the figures shown earlier. It is called a circle.
A circle is a plane closed figure enclosed by a curve, having no sides and no corner (vertex). Every point in the curve is situated at equal distance from a definite point within the closed figure.
The closed figure shown to the above is a circle. It has a point O within it from which the closing curve is equidistant to every point on the circle.
Radius: The distance between the centre and any point situated at the curve closing the circle is called the radius.
Diameter: The line-segment passing through the centre and joining the two points situated at the curve closing the circle is called the diameter.
Circumference: The curve closing the circle is known as circumference. Every point on the circumference is equidistant from the centre.
The plane surfaces (like rectangle, triangle, circle, etc.) are called two dimensional (2-D) figures as they have length and breadth.
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