Circles HELP! (was: Polygons Help)

Ok, I'm back again...

Here are a few polygons sums.

1. An isosceles triangle whose sides are 13 cm, 13cm and 10cm is inscribed in a circle. Find the radius of the circle

2. Two parallel chords of a circle are of lengths 6cm and 8cm respectively and lie on the same side of the center. If the radius of the circle is 5cm, find the distance between the chords.

3. AB and CD are two parallel cords. AB = 10cm, CD = 24 cm. Distance between AB and CD is 17 cm. Find the radius of the circle

See, in the last one, my teacher first tried it out assuming both were on the same side of the center just to explain to us in detail. But I didn't understand one part in it.

If suppose both chords lie on the same side of the center... Then,

Draw a perpendicular. So O (The center of the circle), to P (The bisecting point on the smaller chord AB of the perpendicular ) and O to Q (The bisecting point of the larger chord CD of the perpendicular ).

Now according to Pythagoras' Theorem...

S²+S² = Hypotenuse²

Draw a radius (which is also the hypotenuse) to both the chords...

Therefore,

OQ²+QD² = OD²

Now if you take the radius as x, then you need to take OP as y right?

Since OQ = y + OP

Now if somebody can explain further. I know you can do substitution, but I got stuck in some place. See, I know that this is just gonna be a waste of time, since I KNOW that the chords need to be on different sides of the center, but still, if somebody can explain

Also please help me with the remaining sums... THANKS A LOT GUYS! YOU ROCK! (Please Hurry :p Sorry)