[tex]\displaystyle\\
\text{Aplicam reciproca teoremei lui Pitagora.}\\\\
\sqrt{DC^2 + BC^2}=\\\\
= \sqrt{200^2 + 150^2}= \\\\
= \sqrt{40000 + 22500}= \\\\
= \sqrt{62500 }= 250 = BD\\\\
\Longrightarrow~~m(\sphericalangle BCD = 90^{o})\\\\
\text{D e f i n i t i e: }\\
\text{ \bf Paralelogramul cu un unghi drept este dreptunghi.}\\\\
\Longrightarrow~~\text{Toate unghiurile "paralelogramului" ABCD sunt de } 90^{o}
[/tex]
