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A circular disc of radius b has a hole of radius a at its centre (see figure). If the mass per unit area of the disc varies as ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ σ r 0 , then the radius of gyration of the disc about its axis passing through the centre is: a b

1. 3 b a +
2. 3 ab b a 2 2 + +
3. 2 ab b a 2 2 + +
4. 2 b a +

Correct answer: 3 ab b a 2 2 + +

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