The first few members of the geometric progression are written out: -0.4; 2; -10; … Find the sum of the first five of its members.

The nth term of the geometric progression can be found by the formula:

bn = b1 * q ^ (n – 1), where b1 is the exponent, n is the number of the member.

Then, in a specific example, the equality is true:

2 = (-0.4) * q ^ (2 – 1)

q = 2: (-0.4) = – 5.

The sum of n terms can be found by the formula: Sn = b1 * (1 – q ^ n) / (1 – q).

S5 = (-0.4) * (1 – (-5) ^ 5) / (1 – 5) = (-0.4) * (1 – (-3125) / (-4) = 312.6.