Sigma 15 k = 9 (k+2)​

salahsatu cara

∑₉₋₈¹⁵⁻⁸ (k+10)=

= ∑₁⁷  (k+10)

a= 11

u7 = 17

s7= 7/2 (11 + 17)

s7= 98