Răspuns:
Explicație pas cu pas:
ABCD trapez, OE║AB, ⇒ΔAOE≅ΔACD, ⇒AE/AD=OE/CD.
ΔDEO≅ΔDAB, ⇒EO/AB=DE/DA=
[tex]=\dfrac{AD-AE}{AD}= \dfrac{AD}{AD}-\dfrac{AE}{AD}= 1-\dfrac{AE}{AD}=1-\dfrac{OE}{DC} \\deci~\dfrac{OE}{AB}+ \dfrac{OE}{DC}=1~|:OE,~~\dfrac{1}{AB}+\dfrac{1}{DC}=\dfrac{1}{OE}[/tex]