1÷(2÷3)÷(3÷4)÷(4÷5)÷.÷(1999÷2000)=

The original formula = (3 / 2) × (4 / 3) × (5 / 4) × × (1999 / 1998) × (2000 / 1999) = (1 / 2) × 2000 (about missing in the middle) = 1000

As shown in the figure, D is a point on the hypotenuse BC in RT △ ABC, and BD = AB, make a vertical line of BC through D, intersect AC at e, if AE = 12cm, then the length of De is______ cm．

Connect be. ∵ D is a point on the hypotenuse BC in RT △ ABC, and BD = AB, make the vertical line of BC through D, intersect AC at e, ∵ a = ∵ BDE = 90 °, in RT △ DBE and RT △ Abe, BD = AB (known), be = EB (common edge), ≌ RT △ DBE ≌ RT △ Abe (HL), ≌ AE = ed, and

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