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7.Binomial Theorem
hard
If ${ }^{20} \mathrm{C}_{\mathrm{r}}$ is the co-efficient of $\mathrm{x}^{\mathrm{r}}$ in the expansion of $(1+x)^{20}$, then the value of $\sum_{r=0}^{20} r^{2}\,\,{ }^{20} C_{r}$ is equal to :
A
$420 \times 2^{19}$
B
$380 \times 2^{19}$
C
$380 \times 2^{18}$
D
$420 \times 2^{18}$
(JEE MAIN-2021)
Solution
$\sum_{r=0}^{20} r^{2} \cdot{ }^{20} C_{r}$
$\sum(4(r-1)+r) \cdot{ }^{20} C_{r}$
$\sum r(r-1) \cdot \frac{20 \times 19}{r(r-1)} \cdot{ }^{18} C_{r}+r \cdot \frac{20}{r} \cdot \sum^{19} C_{r-1}$
$\Rightarrow 20 \times 19.2^{18}+20.2^{19}$
$\Rightarrow 420 \times 2^{18}$
Standard 11
Mathematics