Two dimensional shapes and lines are discussed here.
All solids are three dimensional in shapes. Solids are covered by faces. Thus the outer covers of a solid are called faces, i.e., surfaces. These surfaces may be plane or curved. The cuboid, cube etc. have plane surface, while sphere has curved surface. Some solids like cylinder have both plane and curved surfaces.
3-D shaped solids are covered by 2-D surfaces. If any two points are joined, a line segment is formed. In this chapter planes and lines will be discussed.
If the outlines of any face of a three dimensional face are drawn, a two dimensional plane figure is obtained.
Consider the following plane figures
The outline of a face of a cuboid forms a rectangle. It has two pairs of opposite sides i.e., it has four sides. The opposite sides are equal. It has four corners i.e., vertices.
Here the name of rectangle is rect. ABCD.
A, B, C, D are corners or vertices of the rectangle.
Side AB = DC, Side BC = AD.
The outlines of a cube make a square. All its sides are equal to each other.
It has four corners or vertices.
PQRS is the name of the square whose sides PQ = QR = RS = SP and P, Q, R, S are four corners.
A triangle has three sides and three corners.
DEF is a triangle whose sides are DE, EF and DF.
The triangle has three corners or vertices named D, E and F.
The outline of the plane surface of a cone forms a circle.
It has a centre whose distance from any point on the outline is always the same.
This distance is called the radius of the circle.
A circle has no corner and no side.
The surfaces of rectangle, square, triangle and circle are plane surfaces. All the surfaces or faces of cuboid, cube i.e., books, matchbox, etc. are called plane. But the surface of a sphere i.e., ball is curved surface. A cylinder and cone have both curved and plane surfaces.