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## 'Angle in a semicircle is a right angle'[A printable version of this page may be downloaded here.] Fig 1 Fig 2
In Fig 1, BAD is a diameter of the circle, C is a point on the circumference, forming the triangle BCD. The angle BCD is the 'angle in a semicircle'. [Of course, looking at Fig 2, you could just use theorem 1: angle BAD is 180 degrees and is the 'angle at the centre', and BCD is the 'angle at the circumference', so must be half of 180 degrees, ie 90 degrees....] |