A) 6
B) 7
C) 8
D) 9
E) 5
Correct Answer: B
Solution :
\[{{T}_{r}}={{T}_{(r-1)+1}}{{=}^{20}}{{C}_{r-1}}.{{x}^{r-1}}\] \[\therefore \]Coefficient of the r th term\[{{=}^{20}}{{C}_{r-1}}\] and coefficient of the\[(r+1)\]th term \[{{=}^{20}}{{C}_{r}}\] \[\therefore \] \[\frac{^{20}{{C}_{r-1}}}{^{20}{{C}_{r}}}=\frac{1}{2}\] \[\Rightarrow \] \[\frac{r}{21-r}=\frac{1}{2}\] \[\Rightarrow \] \[r=7\]
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