(Bn) – geometric progression, the denominator of the progression is -5 B1 = -5. Find the sum of its first four terms.

The solution of the problem:

1. In order to find the sum of the first four terms of a given geometric progression, we use the following formula:

Sn = b1 (q ^ n -1) / (q -1).

2. Substitute the specified values and calculate the amount:

Sn = -5 ((-5) ^ 4 – 1) / (-5 -1);

Sn = -5 * 624/6;

Sn = 520.

Answer: The sum of the first four members of a given geometric progression is 520.