$a\text{)}P=\frac{1}{2}\cdot \left(1\text{cm}+8\text{cm}\right)\cdot 4\text{cm}=\frac{1}{2}\cdot 9\text{cm}\cdot 4\text{cm}=\frac{1}{{\sout{2}}^1}\cdot {\sout{36}}^{18}{\text{cm}}^2=18{\text{cm}}^2$

$b\text{)}P=\frac{1}{2}\cdot \left(1\text{cm}+3\text{cm}\right)\cdot 2\text{cm}=\frac{1}{{\sout{2}}^1}\cdot {\sout{4}}^2\text{cm}\cdot 2\text{cm}=4{\text{cm}}^2$

$c)P=\frac{1}{2}\cdot \left(8\text{dm}+10\text{dm}\right)\cdot 4\text{dm}=\frac{1}{{\sout{2}}^1}\cdot {\sout{18}}^9\text{dm}\cdot 4\text{dm}=36{\text{dm}}^2$