Extract Coefficients Using Roots of Unity or Substitution Filter

The question asks to find the sum of selected coefficients (e.g., every third coefficient) from a polynomial expansion, typically solved using roots of unity or strategic substitutions.

isi-entrance 2019 Q22 View
Let the integers $a _ { i }$ for $0 \leq i \leq 54$ be defined by the equation
$$\left( 1 + X + X ^ { 2 } \right) ^ { 27 } = a _ { 0 } + a _ { 1 } X + a _ { 2 } X ^ { 2 } + \cdots + a _ { 54 } X ^ { 54 }$$
Then, $a _ { 0 } + a _ { 3 } + a _ { 6 } + a _ { 9 } + \cdots + a _ { 54 }$ equals
(A) $3 ^ { 26 }$
(B) $3 ^ { 27 }$
(C) $3 ^ { 28 }$
(D) $3 ^ { 29 }$.
jee-main 2021 Q66 View
Let $\left( 1 + x + 2 x ^ { 2 } \right) ^ { 20 } = a _ { 0 } + a _ { 1 } x + a _ { 2 } x ^ { 2 } + \ldots + a _ { 40 } x ^ { 40 }$, then $a _ { 1 } + a _ { 3 } + a _ { 5 } + \ldots + a _ { 37 }$ is equal to
(1) $2 ^ { 20 } \left( 2 ^ { 20 } - 21 \right)$
(2) $2 ^ { 19 } \left( 2 ^ { 20 } - 21 \right)$
(3) $2 ^ { 19 } \left( 2 ^ { 20 } + 21 \right)$
(4) $2 ^ { 20 } \left( 2 ^ { 20 } + 21 \right)$