Let $ (1 + x + x^2)^{10} = a_0 + a_1 x + a_2 x^2 + ... + a_{20} x^{20} $. If $ (a_1 + a_3 + a_5 + ... + a_{19}) - 11a_2 = 121k $, then k is equal to _______
In the expansion of $\left( \sqrt{5} + \frac{1}{\sqrt{5}} \right)^n$, $n \in \mathbb{N}$, if the ratio of $15^{th}$ term from the beginning to the $15^{th}$ term from the end is $\frac{1}{6}$, then the value of $^nC_3$ is:
The binomial theorem formula is used in the expansion of any power of a binomial in the form of a series. The binomial theorem formula is