Since, according to the condition, the side face of DD1C1C is square, and DС = 3 cm, then DD1 = DС = 3 cm.
At the base of the parallelepiped lies a rectangle in which we draw the diagonal BD.
Then the triangle BDD1 is rectangular, in which, according to the Pythagorean theorem, we determine the length of the leg BD.
BD ^ 2 = BD1 ^ 2 – DD1 ^ 2 = (√22) ^ 2 – 3 ^ 2 = 22 – 9 = 13.
ВD = √13 cm.
The BCD triangle is also rectangular, then BC ^ 2 = BD ^ 2 – CD ^ 2 = 13 – 9 = 4.
BC = 4 cm.
Answer: The length of the BC rib is 4 cm.
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