${r^{th}}$ term in the expansion of ${(a + 2x)^n}$ is
$\frac{{n(n + 1)....(n - r + 1)}}{{r!}}{a^{n - r + 1}}{(2x)^r}$
$\frac{{n(n - 1)....(n - r + 2)}}{{(r - 1)\,!}}{a^{n - r + 1}}{(2x)^{r - 1}}$
$\frac{{n(n + 1)....(n - r)}}{{(r + 1)!}}{a^{n - r}}{(x)^r}$
None of these
If the coefficients of $x^2$ and $x^3$ are both zero, in the expansion of the expression $(1 + ax + bx^2) (1 -3x)^{t5}$ in powers of $x$, then the ordered pair $(a, b)$ is equal to
Let $m$ be the smallest positive integer such that the coefficient of $x^2$ in the expansion of $(1+x)^2+(1+x)^3+\cdots+(1+x)^{49}+(1+m x)^{50}$ is $(3 n+1)^{51} C_3$ for some positive integer $n$. Then the value of $n$ is
If the coefficients of ${p^{th}}$, ${(p + 1)^{th}}$ and ${(p + 2)^{th}}$ terms in the expansion of ${(1 + x)^n}$ are in $A.P.$, then
If the $6^{th}$ term in the expansion of the binomial ${\left[ {\frac{1}{{{x^{\frac{8}{3}}}}}\,\, + \,\,{x^2}\,{{\log }_{10}}\,x} \right]^8}$ is $5600$, then $x$ equals to
The ratio of the coefficient of $x^{15}$ to the term independent of $x$ in the expansion of ${\left( {{x^2} + \frac{2}{x}} \right)^{15}}$ is