To find area $ A $ use formula:
$$ A = \dfrac{ d_1 \cdot d_2 }{ 2 } $$After substituting $d_1 = 5\, \text{cm}$ and $d_2 = 4\, \text{cm}$ we have:
$$ A = \dfrac{ 5\, \text{cm} \cdot 4\, \text{cm} }{ 2 }$$$$ A = \dfrac{ 20\, \text{cm}^2 }{ 2 } $$$$ A = 10\, \text{cm}^2 $$