7.Binomial Theorem
hard

$(1+x)^{n+2}$ के द्विपद प्रसार में तीन क्रमागत पदों के गुणांकों का योगफल, जो $1: 3: 5$ अनुपात में है, होगा

A

$25$

B

$63$

C

$41$

D

$92$

(JEE MAIN-2023)

Solution

${ }^{n+2} C_{ r -1}:{ }^{n+2} C_{ r }:{ }^{ n +2} C _{ r +1}=1: 3: 5$

$\frac{{ }^{n+2} C_{ r -1}}{{ }^{ n +2} C _{ r }}=\frac{1}{3}$

$n =4 r -3 \ldots \ldots \text { (i) }$

$\frac{{ }^{n+2} C _{ r }}{{ }^{ n +2} C _{ r +1}}=\frac{3}{5}$

$8 r -1=3 n \text {…… (ii) }$

$\text { From, (i) and (ii) }$

$r =2 \text { and } n =5$

$\text { Required sum }=63$

Standard 11
Mathematics

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