A) 18
B) 20
C) 17
D) None of these
Correct Answer: A
Solution :
According to condition \[\Rightarrow \]\[513=\frac{n}{2}\left\{ 2\times 54+(n-1)(-3) \right\}\] \[\Rightarrow \]\[{{a}_{1}}+{{a}_{24}}=75\]\[\Rightarrow \] \[3{{n}^{2}}-111n+1026=0\] \[\Rightarrow \]\[n-2\] Hence\[n=18\].
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