MCQ
$S_n = 2 + 4 + 7 + 11 + ..n$ પદ હોય , તો $t_n$ $= ….$
- A$\frac{{{n^2} + n + 1}}{2}$
- B$n^2 + n + 2$
- ✓$\frac{{{n^2} + n + 2}}{2}$
- D$\frac{{{n^2} + 2n + 2}}{2}$
$S_{n-1} = 2 + 4 + 7+…..(n - 1)$ પદસુધી
હવે, $t_n = S_n - S_{n-1} $
$= 2 + \{2 + 3 + 4 + ….(n - 1)$ પદ$\}$
$= 1 +$ $\{$$1 + 2 + 3 + 4…n$ પદ$\}$
$=1+\frac{n(n+1)}{2}\,\,\,=\frac{{{n}^{2}}+n+2}{2}$
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