O is the centre of circle and tangents from point A and B are parallel
We know that the line joining point of contacts of two parallel tangents (here AB) passes through the centre
And as a line from the centre is perpendicular to tangent, hence that line(AB) will be the distance between parallel tangents
AB passes through centre O hence AB is also the diameter of the circle
Hence the distance between the two parallel tangents will be the diameter of the circle
The distance AB = 10 cm in diameter of the circle
Hence radius will be half of the diameter which is 5 cm
