Rysunek pomocniczy: 

$P_I=\frac{1}{2}\cdot \left(3,5\text{cm}+2,5\text{cm}\right)\cdot 1,5\text{cm}=\frac{1}{{\sout{2}}^1}\cdot {\sout{6}}^3\text{cm}\cdot 1,5\text{cm}=3\text{cm}\cdot 1,5\text{cm}=4,5{\text{cm}}^2$  

$P_{II}=\frac{1}{2}\cdot \left(4\text{cm}+2\text{cm}\right)\cdot 1\text{cm}=\frac{1}{{\sout{2}}^1}\cdot {\sout{6}}^3\text{cm}\cdot 1\text{cm}=3\text{cm}\cdot 1\text{cm}=3{\text{cm}}^2$  

$P_{III}=\frac{1}{2}\cdot \left(2\text{cm}+1\text{cm}\right)\cdot 2\text{cm}=\frac{1}{{\sout{2}}^1}\cdot 3\text{cm}\cdot {\sout{2}}^1\text{cm}=3{\text{cm}}^2$