d = -5.4
a(1) = 1.2
S(5) - ?
a(5) = a(1) + 4d
a(5) = 1.2 + 4 * (-5.4) = - 20.4
S(5) = (a(1) +a(5)) / 2 * n
S(5) = (1.2 - 20.4) /2 * 5 = -19.2 * 2.5 = - 48
d = -5.4
a(1) = 1.2
S(5) - ?
a(5) = a(1) + 4d
a(5) = 1.2 + 4 * (-5.4) = - 20.4
S(5) = (a(1) +a(5)) / 2 * n
S(5) = (1.2 - 20.4) /2 * 5 = -19.2 * 2.5 = - 48