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There are two long co-axial solenoids of same length l . The inner and outer coils have radii r 1 and r 2 and number of turns per unit length n 1 and n 2 , respectively. The ratio of mutual inductance to the self - inductance of the inner-coil is :

1. 1 2 n n . 2 1 2 2 r r
2. 1 2 n n
3. 2 1 n n
4. 1 2 n n . 2 1 r r

Correct answer: 1 2 n n

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