If the waist length of an isosceles triangle with a vertex angle of 120 ° is 4cm, the diameter of its circumscribed circle is smaller______ cm.
As shown in the figure; △ ABC, ∠ ACB = 120 °, AC = BC = 4cm; it is easy to know ∠ OCA = 12 ∠ ACB = 60 °; and ∵ OA = OC, ∵ OAC is equilateral triangle; ∵ OA = OC = AC = 4cm; so the circumscribed circle diameter of isosceles triangle is 8cm
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