Angle in a semicircle: a triangle on a diameter

An example figure from the Circle Generator.

A circle with diameter AB through its center O and a point C on the circle; triangle ABC has a right-angle square at C, the angle at B labeled 55° and the angle at A labeled x — A circle with a diameter AB through its center O and a point C on the upper half. The chords CA and CB make a triangle on the diameter, with a right-angle square at C. The angle at B is marked and labeled 55° and the angle at A is marked and labeled x.

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Answer key

x = 35°

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