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7.Binomial Theorem
normal
$x^r$ occurs in the expansion of ${\left( {{x^3} + \frac{1}{{{x^4}}}} \right)^n}$ provided -
A
$2n -r$ is divisible by $5$
B
$3n -r$ is divisible by $5$
C
$2n -r$ is divisible by $7$
D
$3n -r$ is divisible by $7$
Solution
${{\rm{T}}_{p + 1}} = {\,^{\rm{n}}}{{\rm{C}}_p}{\left( {{{\rm{x}}^3}} \right)^{{\rm{n}} – p}}{\left( {{{\rm{x}}^{ – 4}}} \right)^b} = {\,^n}{{\rm{C}}_p}{{\rm{x}}^{3n – 7p}}$
$\mathrm{r}=3 \mathrm{n}-7 \mathrm{p}$
$\Rightarrow \mathrm{p}=\frac{3 \mathrm{n}-\mathrm{r}}{7}$
Standard 11
Mathematics