A) 3n2
B) 2n2
C) 6n-2n2
D) 6n-n2
Correct Answer: D
Solution :
\[a-d+a+a+d=9\Rightarrow a=3\] \[{{(a-d)}^{2}}+{{a}^{2}}+{{(a+d)}^{2}}=35\] \[\Rightarrow 18+2{{d}^{2}}+9=35\Rightarrow {{d}^{2}}=4\Rightarrow d=\pm 2\] \[S=\frac{n}{2}(2a-2d+nd-d)\] Taking d = -2. we get (\[\because \] d = 2 will not give any option) \[S=\frac{n}{2}[6-2n+6]=6n-{{n}^{2}}\]
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