NCERT Book Class 9 Maths Chapter 11 Surface Area and Volume

SURFACE AREAS AND VOLUMES 137

CHAPTER 11

SURFACE AREAS AND VOLUMES

11.1 Surface Area of a Right Circular Cone
We have already studied the surface areas of cube, cuboid and cylinder. We will now
study the surface area of cone.
So far, we have been generating solids by stacking up congruent figures. Incidentally,
such figures are called prisms. Now let us look at another kind of solid which is not a
prism (These kinds of solids are called pyramids.). Let us see how we can generate
them.
Activity : Cut out a right-angled triangle ABC right angled at B. Paste a long thick
string along one of the perpendicular sides say AB of the triangle [see Fig. 11.1(a)].
Hold the string with your hands on either sides of the triangle and rotate the triangle
about the string a number of times. What happens? Do you recognize the shape that
the triangle is forming as it rotates around the string [see Fig. 11.1(b)]? Does it remind
you of the time you had eaten an ice-cream heaped into a container of that shape [see
Fig. 11.1 (c) and (d)]?

Fig. 11.1