324
252
-252
-324
Correct answer is A
x−3x2=x−3x−2
Let the power on x be t, so that the power on x−2 = 9 - t
(x)t(x−2)9−t=x3⟹t−18+2t=3
3t=3+18=21∴
To obtain the coefficient of x^{3}, we have
^{9}C_{7}(x)^{7}(3x^{-2))^{2} = \frac{9!}{(9 - 7)! 7!}(x)^{7}(9x^{-4})
= \frac{9 \times 8 \times 7!}{7! 2!} \times 9(x^{3}) = 324x^{3}
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