PQ is tangent to outer circle and PR is tangent to inner circle. if PQ=4cm,OQ=3cm and OR=2cm then the length of PR is

a)5cm b)root21

c)4cm d)3cm

visit here to view the FIGURE "sumspics.blogspot.in"

in above website check "p1"

given PQ = 4, OQ = 3, OR= 2

from above figure

triangle OQP is a right angled triangle

note: Tangents touches the circle at right angles

by pythagoras OP²=OQ²+PQ²

OP = 5

similarly OPR is a right angled triangle

OP²=OR²+PR²

25 = 4 + PR²

PR = root (21)