Circle theorems:

• Angles at the centre and circumference:

  • The angle subtended by an arc at the centre is twice the angle subtended at the circumference.

  
• Angles in the same segment:

  • The angles at the circumference subtented by the same arc are equal.

  
• Cyclic quadrilaterals:

  • A quadeilateral drawn inside a circle. Every corner of the quadrilateral must touch the circumference.
  • The opposite anfles add up to 180$^{\circ}$
  • (a+c=180)

  
• Angles in a cemicircle:

  • The angle at the circumference in a semicircle is a right angle.