Question

Evaluate S8 for the series 1+3+6+9+12.........​

Answer 1

S8 is 64

Explanation:

`{ \boxed{ \bf{S_(n) = (n)/(2) [2a + (n - 1)d]}}}`

n is number of terms; n = 8

a is the first term; a = 1

d is the common difference: d = 3-1; d = 2

`{ \sf{S_(8) = (8)/(2)[(2 * 1) + (8 - 1) * 2 ]}} \\ \\ { \sf{{S_(8)} = 4(2 + 14)}} \\ { \sf{S_(8)}} = 4 * 16 \\ { \sf{S_(8)}} = 64`