a(n) - ap пр
a(2) = 4
a(12) = 16
a(7) -?
cистема:
{a(2) = a(1) +d {4 = a(1) + d <=> {a(1) = 4-d {a(1) =2.8
{a(12) = a(1) + 11d <=> {16 = a(1) + 11d {16 = 4-d+11d <=> { d = 1.2
a(7) = a(1) + 6d
a(7) = 2.8 + 6 * 1.2
a(7) = 2.8 + 7.2
a(7)= 10
