a(n) ар пр
a(1) = 5.2
d = 7.2
a(3) - ?
a(n) = a(1) +(n-1)d
a(3) = a(1) + 2d
a(3) = 5.2 + 2 * 7.2
a(3) = 5.2 + 14.4 = 19.6
a1=5,2
a3=a1+d(n-1)
a3=5,2+7,2*2=19,6
