The radius of two concentric circles are 9 cm and 15 cm.

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The radius of two concentric circles are 9 cm and 15 cm. If the chord of the greater circle be a tangent to the smaller circle, then the length of that chord is

1. 24 cm
2. 12 cm
3. 30 cm
4. 18 cm

Correct Option: A

According to question , we draw a figure of a circle with centre O ,

Here , BO = OC = 15 cm and OD = 9 cm.
From ∆ BDO ,
∴ BD = √BO² - OD²
∴ BD = √15² - 9²
BD = √24 × 6 = 12 cm
∴ BC = 2 × 12 = 24 cm.