The first three terms in the expansion of $(1 + ax)^{n}$ $(n \neq 0)$ are $1$, $6x$ and $16x^{2}$. Then the values of $a$ and $n$ are respectively
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Set up equations from the 2nd and 3rd coefficients of $(1+ax)^n$: $na = \text{coeff of }x$ and $\tfrac{n(n-1)}{2}a^2 = \text{coeff of }x^2$. Divide to eliminate one variable.