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7.Binomial Theorem
normal
In the expansion of $(1 + x)^{43}$ if the co-efficients of the $(2r + 1)^{th}$ and the $(r + 2)^{th}$ terms are equal, the value of $r$ is :
A
$12$
B
$13$
C
$14$
D
$15$
Solution
Given in the expansion of $(1+x)^{43}$ the coefficient of $(2 r+1)^{t h}$ and coefficient of $(r+2)^{t h}$ are equal
${ }_{2 r}^{43} C={ }_{r+1}^{43} C$
Either $2 r=r+1 \Rightarrow r=1$ or
$2 r+r+1=43 \Rightarrow r=14$
Hence $r=1,14$
Standard 11
Mathematics