In the circle, O is the centre and AB is a diameter. a) work out the size of angle a

b) work out the size of angle b

Step-by-step explanation:

a). m∠a = 62°

b). m∠b = 118°

Step-by-step explanation:

a). Since, an angle inscribed in a semicircle is a right angle,

  m∠ACB = 90°

  By triangle sum theorem in ΔABC,

  m∠CAB + m∠ACB + m∠ABC = 180°

  28° + 90° + m∠ABC = 180°

  118° + m∠ABC = 180°

  m∠ABC = 180°- 118°

  a = 62°

b). Since, quadrilateral ABCD is a cyclic quadrilateral,

   m∠ADC + m∠ABC = 180°

   b + a = 180°

   b + 62° = 180°

   b = 180° - 62°

   b = 118°