STEP 1: find slant height $ s $

To find slant height $ s $ use Pythagorean Theorem:

$$ h^2 + \frac{ a^2 }{ 4 } = s^2 $$

After substituting $h = 18.2\, \text{cm}$ and $a = 12\, \text{cm}$ we have:

$$ \left( 18.2\, \text{cm} \right)^{2} + \frac{ \left( 12\, \text{cm} \right)^{2} }{ 4 }= s^2 $$ $$ 331.24\, \text{cm}^2 + \frac{ 144\, \text{cm}^2 }{ 4 }= s^2 $$ $$ 331.24\, \text{cm}^2 + 36\, \text{cm}^2 = s^2 $$ $$ s^2 = 367.24\, \text{cm}^2 $$ $$ s = \sqrt{ 367.24\, \text{cm}^2 } $$$$ s = 19.1635\, \text{cm} $$

STEP 2: find lateral surface $ L $

To find lateral surface $ L $ use formula:

$$ L = 2 \cdot a \cdot s $$

After substituting $a = 12\, \text{cm}$ and $s = 19.1635\, \text{cm}$ we have:

$$ L = 24\, \text{cm} \cdot 19.1635\, \text{cm} $$$$ L = 459.9242\, \text{cm}^2 $$

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