А3 = 18-5=13
А2=13-5 = 8
А1 = 8-5 = 3
А14 =А1+(13)*5 = 3+65=68
S14 = (A1+A14)*14 / 2 = (3+68)*7 = 71*7 = 497
А3 = 18-5=13
А2=13-5 = 8
А1 = 8-5 = 3
А14 =А1+(13)*5 = 3+65=68
S14 = (A1+A14)*14 / 2 = (3+68)*7 = 71*7 = 497