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A rectangular piece of cardboard of a breadth of 21 cm is rolled along its length to form a cylinder. The radius of the cylinder so formed is 7 cm. What was the area of cardboard that was used in making the cylinder?

Given,

The radius of the cylinder is given as 7 cm.

the height of the cylinder equals the breadth of the cardboard i.e., 21 cm.

The curved surface area of the cylinder equals the area of the rectangular piece of cardboard.

The curved surface area of the cylinder = 2 × π × radius × height

$$\begin{gathered} =2\times \frac{22}{7}\times 7\times 21 \\ =924 {\mathrm{cm}}^2 \end{gathered}$$

Therefore, the curved surface area of the cylinder = 924 cm²