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20.6: Площа твердих паралелограмів
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Без втрати спільності можна вважати, що $\blacksquare ABCA'$ містить $\blacksquare ABCD$ і $\blacksquare ABB'A'$ . \[\begin{aligned} \text{area }( \blacksquare ABCD)&+\text{area }(\blacktriangle AA'...Без втрати спільності можна вважати, що $\blacksquare ABCA'$ містить $\blacksquare ABCD$ і $\blacksquare ABB'A'$ . $\begin{aligned} \text{area }( \blacksquare ABCD)&+\text{area }(\blacktriangle AA'D)= \\ &= \text{area }(\blacksquare ABB'A')+ \text{area } (\blacktriangle BB'C). \end{aligned}$ $\begin{array} {l} {\text{area } (\blacksquare AB'ED) = \text{area } (\blacksquare ABCD),} \\ {\text{area } (\blacksquare AB'ED) = \text{area } (\blacksquare AB'C'D')} \end{array}$