Answer 1

Let the centre of the circle be O.
AB = 12 cm
OC = 12 cm
OC ⊥ AB
We know perpendicular drawn from the centre of the circle to the chord, bisects the chord.
So, AC = 6 cm
In   `square` OCA, 

OC² + AC² = AO²

⇒ 8² + 6² = AO²

⇒ AO² = 100

⇒ AO = 10

Thus, AO = 10 cm
Diameter = 2 × 10 cm = 20 cm.