5+13+21+….+181=…

181 = 5 + ( n-1 ) 8
181 = 5 + 8n – 8
181 = 8n – 3
8n = 184
n = 23

S23 = 23/2 (2×5+(23-1)8)
S23 = 23/2 x 186
S23 = 2.139