JEE MAIN - Mathematics (2019 - 12th April Morning Slot - No. 20)
The coefficient of x18 in the product
(1 + x) (1 – x)10 (1 + x + x2)9 is :
(1 + x) (1 – x)10 (1 + x + x2)9 is :
126
- 84
- 126
84
Explanation
Coefficient of x18 in (1 + x) (1 - x)10 (1 + x + x2)9
$$ \Rightarrow $$ Coefficient of x18 in {(1 - x) (1 - x2) (1 + x + x2)}9
$$ \Rightarrow $$ Coefficient of x18 in (1 - x2) (1 - x3)9
$$ \Rightarrow $$ 9C6 - 0 = 84
$$ \Rightarrow $$ Coefficient of x18 in {(1 - x) (1 - x2) (1 + x + x2)}9
$$ \Rightarrow $$ Coefficient of x18 in (1 - x2) (1 - x3)9
$$ \Rightarrow $$ 9C6 - 0 = 84
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