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If the coefficient of x 15 in the expansion of 15 3 / 1 3 bx 1 ax        is equal to the coefficient of x – 15 in the expansion of 15 3 3 / 1 bx 1 ax        , where a and b are positive real numbers, then for each such ordered pair ( a , b )

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

Correct answer: ab = 1

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