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$\small\text{Lato obliquo } l= \dfrac{5}{6}b = \dfrac{5}{\cancel6_1}·\cancel{42}^7 = 5·7 = 35\,cm;$
$\small\text{altezza } h= \sqrt{l^2-\left(\dfrac{b}{2}\right)^2} = \sqrt{35^2-\left(\dfrac{42}{2}\right)^2} = \sqrt{35^2-21^2} = 28\,cm\; \text{(teorema di Pitagora)};$
$\small\text{area } A= \dfrac{b·h}{2} = \dfrac{42·\cancel{28}^{14}}{\cancel2_1} = 42·14 = 588\,cm^2.$