Since, by condition, the sum of the sides is 20 cm, then AB + CD = 20 cm.
The perimeter of the trapezoid is: Ravsd = (AB + CD) + BC + AD, then:
(BC + AD) = Ravsd – (AB + CD) = 50 – 20 = 30 cm.
The middle line of the trapezoid is: KM = (BC + AD) / 2 = 30/2 = 15 cm.
Answer: The length of the middle line is 15 cm.
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