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The radius of two concentric circles are 17cm and 10cm. A straight line ABCD intersects the larger circle at the point A and D and intersects the smaller circle at the points B and C. If BC = 12 cm, then the length of AD (in cm) is :
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- 20
- 24
- 30
- 34
- 20
Correct Option: C
As per the given in question , we draw a figure of two concentric circles whose radii are 17cm and 10cm 
Here , BE = EC = 6 cm, OB = 10 cm, OA = 17 cm
From ∆ OBE,
OE = √OB² - BE²
OE = √10² - 6² = √16 × 4 = 8 cm
From ∆ OAE,
AE = √OA² - OE²
AE = √17² - 8²
AE = √25 × 9 = 15 cm
∴ AD = 2 × AE = 2 × 15 = 30 cm
