Question

find the sum of first 20 terms of a thematic series 2 + 7 + 12 +​

Answer 1

$\sf\underline{Question:}$

Find the sum of first 20 terms of arithmetic series 2, 7, 12..

$\sf\underline{Answer:}$

In the given AP, first term (a) is 2

common difference (d) = 7-2 = 5

Sum of 'n' terms in an AP is given by

$\boxed{S _{n} = \frac{n}{2} [2a + (n - 1)d]}$

Here 'n' is 20 [required in the question]

$S _{20} = \frac{20}{2} [2(2) + (20 - 1)(5)]$

$S _{20} = 10 [4 + 95]$

$S _{20} = 10 \times 99 = 990$

$\boxed{S _{20} = 990}$

Sum of first 20 terms of the given AP is 990

Have a good evening ^^