STEP 1: find side $ l $
To find side $ l $ use Pythagorean Theorem:
$$ r^2 + h^2 = l^2 $$After substituting $ r = 3.9 $ and $ h = 12 $ we have:
$$ 3.9^2 + 12^2 = l^2 $$ $$ 15.21 + 144 = l^2 $$ $$ l^2 = 159.21 $$ $$ l = \sqrt{ 159.21 } $$$$ l = 12.6178 $$STEP 2: find Curved Surface Area $ CSA $
To find Curved Surface Area $ CSA $ use formula:
$$ CSA = r \cdot l \cdot \pi$$After substituting $ r = 3.9 $ and $ l = 12.6178 $ we have:
$$ CSA = 3.9 \cdot 12.6178 \cdot \pi$$$$ CSA = 3.9 \cdot 12.6178 \cdot \pi $$$$ CSA = 49.2096\pi $$