To find diagonal $ d1 $ use formula:

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

After substituting $d_2 = 8\, \text{mm}$ and $a = 7\, \text{mm}$ we have:

$$ d_1 ^ {\,2} + \left( 8\, \text{mm} \right)^{2} = 4 \cdot \left( 7\, \text{mm} \right)^{2} $$ $$ d_1 ^ {\,2} + 64\, \text{mm}^2 = = 196\, \text{mm}^2 $$ $$ d_1 ^ {\,2} = = 196\, \text{mm}^2 - 64\, \text{mm}^2 $$ $$ d_1 ^ {\,2} = 132\, \text{mm}^2 $$ $$ d_1 = \sqrt{ 132\, \text{mm}^2 } $$$$ d_1 = 2 \sqrt{ 33 }\, \text{mm} $$

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