A) 5
B) 10
C) 12
D) 14
Correct Answer: D
Solution :
\[S=\frac{n}{2}[2a+(n-1)d]\] Þ \[406=\frac{n}{2}\left[ 6+(n-1)4 \right]\]Þ \[812=n\,[6+4n-4]\] Þ \[812=2n+4{{n}^{2}}\] Þ \[406=2{{n}^{2}}+n\] Þ \[2{{n}^{2}}+n-406=0\] Þ \[\Rightarrow \]\[=\frac{-1\pm \sqrt{3249}}{4}\]\[\frac{3}{7}\] Taking (+) sign, \[(a-d)a=24\].
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