STEP 1: find side $ l $

To find side $ l $ use Pythagorean Theorem:

$$ r^2 + h^2 = l^2 $$

After substituting $r = 20\, \text{cm}$ and $h = 99\, \text{cm}$ we have:

$$ \left( 20\, \text{cm} \right)^{2} + \left( 99\, \text{cm} \right)^{2} = l^2 $$ $$ 400\, \text{cm}^2 + 9801\, \text{cm}^2 = l^2 $$ $$ l^2 = 10201\, \text{cm}^2 $$ $$ l = \sqrt{ 10201\, \text{cm}^2 } $$$$ l = 101\, \text{cm} $$

STEP 2: find Curved Surface Area $ CSA $

To find Curved Surface Area $ CSA $ use formula:

$$ CSA = l \cdot r \cdot \pi$$

After substituting $r = 20\, \text{cm}$ and $l = 101\, \text{cm}$ we have:

$$ CSA = 101\, \text{cm} \cdot 20\, \text{cm} \cdot \pi$$$$ CSA = 101\, \text{cm} \cdot 20\, \text{cm} \cdot \pi $$$$ CSA = 2020\pi\, \text{cm}^2 $$

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