214 MATHEMATICS
13.3 Surface Area of a Right Circular Cylinder
If we take a number of circular sheets of paper and stack them up as we stacked up
rectangular sheets earlier, what would we get (see Fig. 13.6)?
Fig. 13.6
Here, if the stack is kept vertically up, we get what is called a right circular
cylinder, since it has been kept at right angles to the base, and the base is circular. Let
us see what kind of cylinder is not a right circular cylinder.
In Fig 13.7 (a), you see a cylinder, which
is certainly circular, but it is not at right angles
to the base. So, we can not say this a right
circular cylinder.
Of course, if we have a cylinder with a
non circular base, as you see in Fig. 13.7 (b),
then we also cannot call it a right circular
cylinder.
Remark : Here, we will be dealing with only right circular cylinders. So, unless stated
otherwise, the word cylinder would mean a right circular cylinder.
Now, if a cylinder is to be covered with coloured paper, how will we do it with the
minimum amount of paper? First take a rectangular sheet of paper, whose length is
just enough to go round the cylinder and whose breadth is equal to the height of the
cylinder as shown in Fig. 13.8.
Fig. 13.7