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  • If in the expansion of , the coefficients of <img alt="x^{r}" src="https://entr

If in the expansion of (1+x)^{21}, the coefficients of x^{r} and x^{r+1}  be equal, then r is equal to 

Option: 1

9


Option: 2

10


Option: 3

11


Option: 4

12


Answers (1)

best_answer

T_{r+1}={ }^{21} C_r(1)^{21-r}(x)^r={ }^{21} C_r

\therefore Coefficient of x^{r}={ }^{21} C_r and coefficient of x^{r+1}={ }^{21} C_{r+1}

So, we must have { }^{21} C_r={ }^{21} C_{r+1} \Rightarrow r=10

 

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avinash.dongre

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