To find side $ a $ use Pythagorean Theorem:

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

After substituting $s = 51.85\, \text{cm}$ and $e = 57.5\, \text{cm}$ we have:

$$ \left( 51.85\, \text{cm} \right)^{2} + \frac{ a^2 }{ 4 } = \left( 57.5\, \text{cm} \right)^{2} $$ $$ \frac{ a^2 }{ 4 } = \left( 57.5\, \text{cm} \right)^{2} - \left( 51.85\, \text{cm} \right)^{2} $$ $$ \frac{ a^2 }{ 4 } = 3306.25\, \text{cm}^2 - 2688.4225\, \text{cm}^2 $$ $$ a^2 = 617.8275\, \text{cm}^2 \cdot 4 $$ $$ a^2 = 2471.31\, \text{cm}^2 $$ $$ a = \sqrt{ 2471.31\, \text{cm}^2 } $$$$ a = 49.7123\, \text{cm} $$

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