Binomial Theorem question bank (Maths)

If $(1 - x^3)^{10} = \sum\limits_{r=0}^{10} a_r x^r (1-x)^{30-2r}$, then $\dfrac{9a_9}{a_{10}}$ is equal to __________.

The sum of all possible values of $\mathrm{n} \in \mathbf{N}$, so that the coefficients of $x, x^{2}$ and $x^{3}$ in the expansion of $\left(1+x^{2}\right)^{2}(1+x)^{\mathrm{n}}$, are in arithmetic progression is :