To find side $ r $ use Pythagorean Theorem:

$$ r^2 + h^2 = l^2 $$

After substituting $h = 34.5\, \text{cm}$ and $l = 40\, \text{cm}$ we have:

$$ r ^ {\,2} + \left( 34.5\, \text{cm} \right)^{2} = \left( 40\, \text{cm} \right)^{2} $$ $$ r ^ {\,2} = \left( 40\, \text{cm} \right)^{2} - \left( 34.5\, \text{cm} \right)^{2} $$ $$ r ^ {\,2} = 1600\, \text{cm}^2 - 1190.25\, \text{cm}^2 $$ $$ r ^ {\,2} = 409.75\, \text{cm}^2 $$ $$ r = \sqrt{ 409.75\, \text{cm}^2 } $$$$ r = 20.2423\, \text{cm} $$

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