Two parallel chords AB and CD are 3.9 cm a part and lie on the opposite sides of the centre of a circle. If AB= 1.4 cm and CD=4 cm. The radius of the circle is

The perpendicular from the center bisects both chords, so the distances from the center to AB and CD are 0.7 cm and 2.0 cm respectively. Because the chords are on opposite sides of the center and 3.9 cm apart, we find the radius using the right triangle for each chord, ensuring the sum of their distances from the center is 3.9 cm. The radius r satisfies sqrt(r^2 - 0.7^2) + sqrt(r^2 - 2^2) = 3.9, which yields r = 2.5 cm.

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