Hey, I have a few problems with these questions relating to the geometry of a circle and the 4 theorems.

1) In a circle, O is the centre of a circle. C is the midpoint of chord AB. If AB = 10 and OC = 3, find the radius of the circle to 3 sig.fig.

2) In a circle, AB and CD are two parallel chords of length 6cm and 8cm respectively. If the radius is 5, find the distance between the two chords.

3) In a circle, O is the centre of the circle. The radius is 5cm. PQ and RS are two perpendicular chords of the circle, intersecting at K. If each chord is 8cm, find the length of OK.

4) In the circle, AC is the diameter of the circle. BD is perpendicular to AC and cuts AC at E. Prove triangle CBD is an isosceles triangle.

Thanks for any help