# In the binomial expansion of $(1+ax)^n$, the first three terms are $1+12x+64x^2$, then $n=?$

$\begin{array}{1 1} 6 \\ 9 \\ 10 \\ 12 \end{array}$

Given first three terms in the expansion $(1+ax)^n$ are $1,12x,64x^2$
$\Rightarrow\:^nC_1.a=12\:\:and\:\:^nC_2.a^2=64$