Department of Pre-University Education, KarnatakaPUC Karnataka Science Class 11

# If the Coefficients of Three Consecutive Terms in the Expansion of (1 + X)N Be 76, 95 and 76, Find N. - Mathematics

If the coefficients of three consecutive terms in the expansion of (1 + x)n be 76, 95 and 76, find n.

#### Solution

$\text{ Suppose r, } \left( r + 1 \right) \text{ and } \left( r + 2 \right) \text{ are three consecutive terms in the given expansion } .$

$\text { The coefficients of these terms are } ^{n}{}{C}_{r - 1} , ^{n}{}{C}_r \text{ and } ^ {n}{}{C}_{r + 1} .$

$\text{ According to the question, }$

$^{n}{}{C}_{r - 1} = 76$

$^{n}{}{C}_r = 95$

$^{n}{}{C}_{r + 1} = 76$

$\Rightarrow ^{n}{}{C}_{r - 1} = ^{n}{}{C}_{r + 1}$

$\Rightarrow r - 1 + r + 1 = n [\text{ If } ^{n}{}{C}_r = ^{n}{}{C}_s \Rightarrow r = s \text{ or } r + s = n]$

$\Rightarrow r = \frac{n}{2}$

$\therefore \frac{^{n}{}{C}_r}{^{n}{}{C}_{r - 1}} = \frac{95}{76}$

$\Rightarrow \frac{n - r + 1}{r} = \frac{95}{76}$

$\Rightarrow \frac{\frac{n}{2} + 1}{\frac{n}{2}} = \frac{95}{76}$

$\Rightarrow 38n + 76 = \frac{95n}{2}$

$\Rightarrow \frac{19n}{2} = 76$

$\Rightarrow n = 8$

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#### APPEARS IN

RD Sharma Class 11 Mathematics Textbook
Chapter 18 Binomial Theorem
Exercise 18.2 | Q 31 | Page 40