A) \[10\]
B) \[11\]
C) \[12\]
D) \[13\]
Correct Answer: B
Solution :
Let sum of 2n terms of the \[AP\,\,\,57,59,61,63\] ?is \[{{S}_{n}}\]. \[\therefore \] \[{{S}_{n}}=\frac{n}{2}[2\times 57+(n-1)2]\] \[\frac{n}{2}(2n+112)\] According to question \[{{S}_{2n}}={{S}_{n}}\] \[\Rightarrow \] \[n(6n+1)=\frac{n}{2}(2n+112)\] \[\Rightarrow \] \[12n+2=2n+112\] \[\Rightarrow \] \[10n=110\] \[\Rightarrow \] \[n=11\]
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