Find the sum of the first 29 terms of an AS if its $\rm 15^{th}$ term is 2.
Solution
The general term for an Arithmetic Series is given by $\rm t_{n} = a + (n - 1)d$, where the symbols have their usual meanings.
The sum of the first n terms of an Arithmetic Series is given by $\rm S_{n} = \frac{n}{2} \cdot (2a + (n- 1) d)$.
Given,
$\rm t_{15} = 2$
$\rm or, a + (15 - 1) d = 2$
$\rm or, a + 14d = 2$
To find: $\rm S_{29} = ?$
$\rm or, S_{29} = \frac{29}{2} \cdot \left( 2 \cdot a + (29 - 1) d \right )$
$\rm = \frac{29}{2} \left ( 2a + 28 d \right )$
$\rm = \frac{29}{2} \left ( 2 (a + 14d ) \right )$
$\rm = \frac{29}{2} \cdot 2 \cdot (a + 14d)$
$\rm = 29 \cdot (a + 14d)$
From above, we have $\rm a + 14d = 2$. On substituting the value of this expression, we get,
$\rm = 29 \cdot 2$
$\rm \therefore S_{29} = 58$
Hence, the required sum of the first 29 terms of the given Arithmetic Series is 58.