A) 12
B) 10
C) 8
D) 6
Correct Answer: D
Solution :
Since the coefficient of \[{{(r+1)}^{th}}\] term in the expansion of \[{{(1+x)}^{n}}\,\,=\,{{\,}^{n}}{{C}_{r}}\] \[\therefore \,in the expansion of\,{{\left( 1+x \right)}^{18}}\] coefficient of \[{{(2r+4)}^{th}}\,term\,\,=\,{{\,}^{18}}{{C}_{2r\,+\,3}}\] Similarly, coefficient \[{{(r-2)}^{th}}\] term in the expansion of \[{{(1+x)}^{18}}={{\,}^{18}}{{C}_{r-3}}\] If \[^{n}{{C}_{r}}={{ }^{n}}{{C}_{s}}\,\,then\,\,r+s=n\] \[2r+3+r-3=18\] \[\Rightarrow \,\,\,3r=18\Rightarrow \,\,r=6.\]
You need to login to perform this action.
You will be redirected in
3 sec
