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Let m and n be the coefficients of seventh and thirteenth terms respectively in the expansion of 18 1 3 2 3 1 1 x 3 2x ⎛ ⎞ ⎜ ⎟ + ⎜ ⎟ ⎜ ⎟ ⎝ ⎠ . Then 1 3 n m ⎛ ⎞ ⎜ ⎟ ⎝ ⎠ is :

1. 4 9
2. 1 4
3. 9 4
4. 1 9

Correct answer: 9 4

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