Find the sum of the first sixteen terms of the arithmetic progression a1 = -13.4 a3 = -3.8.

a (1) = -13.4, a (2), a (3) = -3.8 – arithmetic progression. We need to find S (16).

Since a (3) = a (1) + 2d, then

d = (a (3) – a (1)) / 2 = (-3.8 + 13.4) / 2 = 4.8.

Let’s find the 16th term according to the general formula:

a (16) = a (1) + 15d = -13.4 + 15 4.8 = 58.6.

Let’s find the sum S (16):

S (16) = (a (1) + a (16)) / 216 = (-13.4 + 58.6) / 216 = 361.6.