STEP 1: find side $ b $

To find side $ b $ use Pythagorean Theorem:

$$ a^2 + b^2 = d^2 $$

After substituting $a = 4.5\, \text{cm}$ and $d = 20.5\, \text{cm}$ we have:

$$ \left( 4.5\, \text{cm} \right)^{2} + b^2 = \left( 20.5\, \text{cm} \right)^{2} $$ $$ b^2 = \left( 20.5\, \text{cm} \right)^{2} - \left( 4.5\, \text{cm} \right)^{2} $$ $$ b^2 = 420.25\, \text{cm}^2 - 20.25\, \text{cm}^2 $$ $$ b^2 = 400\, \text{cm}^2 $$ $$ b = \sqrt{ 400\, \text{cm}^2 } $$$$ b = 20\, \text{cm} $$

STEP 2: find area $ A $

To find area $ A $ use formula:

$$ A = a \cdot b $$

After substituting $a = 4.5\, \text{cm}$ and $b = 20\, \text{cm}$ we have:

$$ A = 4.5\, \text{cm} \cdot 20\, \text{cm} $$$$ A = 4.5\, \text{cm} \cdot 20\, \text{cm} $$$$ A = 90\, \text{cm}^2 $$

Rectangle Calculator