A) 5
B) 7
C) 6
D) 4
Correct Answer: B
Solution :
Clearly, \[^{n}{{C}_{3}}\,={{T}_{n}}.\] So, \[^{n+1}{{C}_{3}}\,-{{\,}^{n}}{{C}_{3}}\,=21\] Þ \[(\,{{\,}^{n}}{{C}_{3}}\,+{{\,}^{n}}{{C}_{2}}\,)\,-{{\,}^{n}}{{C}_{3}}\,=21\] \ \[^{n}{{C}_{2}}\,=\,21\] or\[n(n-1)=42=7\,.\,6\,\,\,\,\therefore \,\,\,n=7\].You need to login to perform this action.
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