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At one end A of a diameter AB of a circle of radius 5 cm, tangent XAY is drawn to the circle. The length of the chord CD parallel to XY and at a distance 8 cm from A is

(A) 4 cm                      (B) 5 cm                                  (C) 6 cm                      (D) 8 cm

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Answer (D) 8 cm
Solution 
According to questions
                

Given AO = OB = 5 cm
Distance between XY and CD = 8 cm
Since D is the center of the circle and CD is chord. If we join OD it become the radius of circle that is OD = 5cm
AZ = 8 cm (given)
AZ = AO + OZ
8 = 5 + OZ
OZ = 3cm
In \bigtriangleupODZ, use Pythagoras theorem
\left ( OD \right )^{2}= \left ( OZ \right )^{2}+\left ( ZD \right )^{2}
\left ( 5 \right )^{2}= \left ( 3 \right )^{2}+\left ( ZD \right )^{2}
\left ( ZD \right )^{2}= 25-9
ZD= \sqrt{16}= 4cm
CD= CZ+ZD
CD= ZD+ZD  \left ( \because CZ= ZD \right )
CD= 2ZD= 2\left ( 4 \right )= 8cm
Length of chord CD = 8 cm

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