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Mathematics · 2022 · 2 marks
CBSE 2022 · Region 1 · Set 1 · Q1
(a) Find the sum of first 30 terms of AP: - 30, - 24, - 18,......
(a)
Find the sum of first $\displaystyle 30$ terms of AP : - $\displaystyle 30$, - $\displaystyle 24$, - $\displaystyle 18$, ..... .
OR
(b)
In an AP if $\displaystyle \mathrm{S}_{\mathrm{n}}=\mathrm{n}(4 \mathrm{n}+1)$, then find the AP.
Marking-scheme solution
\[\begin{aligned}
& a=S_{1}=1(4 \times 1+1)=5 \\
& a+(a+d)=S_{2}=2(4 \times 2+1)=18 \\
& \therefore d=8
\end{aligned}
\]
Hence, AP is
$\displaystyle 5$, $\displaystyle 13$, 21...
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