(i)[2]
Find the first $3$ terms of the expansion of $(1+ax)^5$ in ascending powers of $x$.
(ii)[2]
Since the expansion of $(1-2x)(1+ax)^5$ has no term in $x$, determine the value of the constant $a$.
(iii)[3]
With this value of $a$, determine the coefficient of $x^2$ in the expansion of $(1-2x)(1+ax)^5$.