To find side $ a $ use formula:

$$ d_1^2 + d_2^2 = 4 \cdot a^2 $$

After substituting $d_1 = 12.125\, \text{cm}$ and $d_2 = 7.9375\, \text{cm}$ we have:

$$ \left( 12.125\, \text{cm} \right)^{2} + \left( 7.9375\, \text{cm} \right)^{2} = 4 \cdot a^2 $$ $$ 147.0156\, \text{cm}^2 + 63.0039\, \text{cm}^2 = 4 \cdot a^2 $$ $$ 4 \cdot a^2 = 210.0195\, \text{cm}^2 $$ $$ a^2 = \frac{ 210.0195\, \text{cm}^2 }{ 4 } $$ $$ a^2 = 52.5049\, \text{cm}^2 $$ $$ a = \sqrt{ 52.5049\, \text{cm}^2 } $$$$ a = 7.246\, \text{cm} $$

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