The coefficient of t^4 in the expansion of


The coefficient of $\mathrm{t}^{4}$ in the expansion of $\left(\frac{1-t^{6}}{1-t}\right)^{3}$ is

  1. 12

  2. 15

  3. 10

  4. 14

Correct Option: , 2



$\left(1-t^{18}-3 t^{6}+3 t^{12}\right)(1-t)^{-3}$

$\Rightarrow$ cofficient of $t^{4}$ in $(1-t)^{-3}$ is

${ }^{3+4-1} C_{4}=6 C_{2}=15$

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