7.Binomial Theorem
hard

यदि $(1+ x )^{20}$ के प्रसार में $x ^{ r }$ का गुणांक ${ }^{20} C _{ I }$ है, तो $\sum_{ r =0}^{20} I ^{2}{ }^{20} C _{ I }$ का मान बराबर है.....।

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

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