$${2^{3n}} - 7n - 1$$ is divisible by

Number of terms in the binomial expansion of $$(x+a)^{53}+(x-a)^{53}$$ is

The coefficient of $$x^{10}$$ in the expansion of $$1+(1+x)+...+(1+x)^{20}$$ is

Middle term in the expansion of $${\left( {{x^2} + {1 \over {{x^2}}} + 2} \right)^n}$$ is