STEP 1: find side $ a $
To find side $ a $ use formula:
$$ r = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$After substituting $r = 5\, \text{cm}$ we have:
$$ 5\, \text{cm} = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$ $$ \sqrt{ 3 } \cdot a = 5\, \text{cm} \cdot 2 $$ $$ \sqrt{ 3 } \cdot a = 10\, \text{cm} $$ $$ a = \dfrac{ 10\, \text{cm} }{ \sqrt{ 3 } } $$ $$ a = \frac{ 10 \sqrt{ 3}}{ 3 }\, \text{cm} $$STEP 2: find area $ A $
To find area $ A $ use formula:
$$ A = \dfrac{ 3 \sqrt{ 3 } \cdot a^2 }{ 2 } $$After substituting $a = \dfrac{ 10 \sqrt{ 3}}{ 3 }\, \text{cm}$ we have:
$$ A = \dfrac{ 3 \sqrt{ 3 } \cdot \left( \frac{ 10 \sqrt{ 3}}{ 3 }\, \text{cm} \right)^{2} }{ 2 } $$ $$ A = \dfrac{ 3 \sqrt{ 3 } \cdot \frac{ 100 }{ 3 }\, \text{cm}^2 }{ 2 }$$ $$ A = \dfrac{ 100 \sqrt{ 3 }\, \text{cm}^2 }{ 2 }$$ $$ A = 50 \sqrt{ 3 }\, \text{cm}^2 $$