A) 9
B) 10
C) 11
D) 12
Correct Answer: B
Solution :
\[{{T}_{r+1}}={{\,}^{21}}{{C}_{r}}{{(1)}^{21-r}}{{(x)}^{r}}={{\,}^{21}}{{C}_{r}}\] \[\therefore \]Coefficient of \[{{x}^{r}}={{\,}^{21}}{{C}_{r}}\] and coefficient of \[{{x}^{r+1}}={{\,}^{21}}{{C}_{r+1}}\] So, we must have\[^{21}{{C}_{r}}={{\,}^{21}}{{C}_{r+1}}\Rightarrow r=10\].You need to login to perform this action.
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