To find side $ r $ use Pythagorean Theorem:

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

After substituting $h = 22.5\, \text{cm}$ and $l = 45\, \text{cm}$ we have:

$$ r ^ {\,2} + \left( 22.5\, \text{cm} \right)^{2} = \left( 45\, \text{cm} \right)^{2} $$ $$ r ^ {\,2} = \left( 45\, \text{cm} \right)^{2} - \left( 22.5\, \text{cm} \right)^{2} $$ $$ r ^ {\,2} = 2025\, \text{cm}^2 - 506.25\, \text{cm}^2 $$ $$ r ^ {\,2} = 1518.75\, \text{cm}^2 $$ $$ r = \sqrt{ 1518.75\, \text{cm}^2 } $$$$ r = 38.9711\, \text{cm} $$

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