Connect points A, B, C, D (ends of diameters). We get a quadrilateral with diagonals AC and BD, which intersect at point O.
Consider the triangle ABC.
∠ABС = 90 °, as an inscribed angle in a circle, based on the diameter of the AC.
∠ADC = 90 °, also based on the AC diameter.
Similarly, we obtain ∠BCD = ∠BAD = 90 °, as the angles inscribed in a circle based on the diameter BD.
ABCD is a quadrangle with all angles straight, i.e. ABCD is a rectangle.
This means that its opposite sides are parallel, that is, AB || CD and AD || BC.
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