In a circle two equal and parallel chords are 40" " cm apart and lie on the opposite sides of the centre of the circle. If the length of each chord is 9" " cm, then the radius of the circle is:

For equal chords at distances a and b from centre: r-squared - a-squared = (L/2)-squared and r-squared - b-squared = (L/2)-squared, where L = 9 cm. So a-squared = b-squared, meaning a = b = 20 (since they're on opposite sides and 40 cm apart). Then r-squared = (9/2)-squared + 20-squared = 20.25 + 400 = 420.25. Therefore r = 20.5 cm. The chord length formula is L = 2sqrt(r-squared - d-squared) where d is distance from centre.