1. Angle at centre: Angle at centre = 2 × angle at circumference, standing on the same arc.
2. Angle in a semicircle: Angle in a semicircle = 90°.
3. Angles in the same segment: Angles in the same segment are equal.
4. Cyclic quadrilateral: Opposite angles add to 180°.
5. Tangent-radius: Tangent is perpendicular to radius at point of contact.
6. Two tangents: Tangents from an external point are equal in length.
Remember: Always state the theorem as your reason.
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O is the centre of the circle. Find x, giving reasons:Angle at centre = 100°, angle at circumference on same …
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