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If the term independent of x in the expansion of ( ax2 + 12x3 )10 is 105, then a2 is equal to :
(2) 4
Tr + 1 = 10Cr ( ax2 )10 - r ( 12x3 )r
Power of x: 2(10 - r) - 3r = 20 - 5r
Independent of x, so 20 - 5r = 0, i.e. r = 4
T5 = 10C4 ( a )6 124 = 210a316 = 105
a3 = 8 a = 2
∴ a2 = 4
Answer: (2)
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