It does not need to be. It can be smaller or larger.
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An angle subtended at the semicircular arc is 90 degrees.
The area is 0.5*pi*r2 where r is the radius. The angle is totally irrelevant since it will always by 180 degrees for a semicircle!
To draw an adjacent angle you should draw an angle and inside that angle draw a semi-line. Make sure that both angles are not equal.
The angles of a semi-circle are on a straight line and angles on a straight line add up to 180 degrees.
bisect a angle AOB 35