Les valeurs changes pas a chaque fois. Vous comprendrais mieux avec le code.
Voici mon code:

for (int i = 1; i <= 100; i++)
{
double a =(20*Math.log10((1/(Math.sqrt(1+(i/5))))));
System.out.println("gain= "+a);
}

Ce qui donne:

gain= 0.0
gain= 0.0
gain= 0.0
gain= 0.0
gain= 0.0
gain= 0.0
gain= 0.0
gain= 0.0
gain= 0.0
gain= -3.0102999566398125
gain= -3.0102999566398125
gain= -3.0102999566398125
gain= -3.0102999566398125
gain= -3.0102999566398125
gain= -3.0102999566398125
gain= -3.0102999566398125
gain= -3.0102999566398125
gain= -3.0102999566398125
gain= -3.0102999566398125
gain= -4.771212547196623
gain= -4.771212547196623
gain= -4.771212547196623
gain= -4.771212547196623
gain= -4.771212547196623
gain= -4.771212547196623
gain= -4.771212547196623
gain= -4.771212547196623
gain= -4.771212547196623
gain= -4.771212547196623
gain= -6.020599913279624
gain= -6.020599913279624
gain= -6.020599913279624
gain= -6.020599913279624
gain= -6.020599913279624
gain= -6.020599913279624
gain= -6.020599913279624
gain= -6.020599913279624
gain= -6.020599913279624
gain= -6.020599913279624
gain= -6.989700043360188
gain= -6.989700043360188
gain= -6.989700043360188
gain= -6.989700043360188
gain= -6.989700043360188
gain= -6.989700043360188
gain= -6.989700043360188
gain= -6.989700043360188
gain= -6.989700043360188
gain= -6.989700043360188
gain= -7.781512503836435
gain= -7.781512503836435
gain= -7.781512503836435
gain= -7.781512503836435
gain= -7.781512503836435
gain= -7.781512503836435
gain= -7.781512503836435
gain= -7.781512503836435
gain= -7.781512503836435
gain= -7.781512503836435
gain= -8.45098040014257
gain= -8.45098040014257
gain= -8.45098040014257
gain= -8.45098040014257
gain= -8.45098040014257
gain= -8.45098040014257
gain= -8.45098040014257
gain= -8.45098040014257
gain= -8.45098040014257
gain= -8.45098040014257
gain= -9.030899869919436
gain= -9.030899869919436
gain= -9.030899869919436
gain= -9.030899869919436
gain= -9.030899869919436
gain= -9.030899869919436
gain= -9.030899869919436
gain= -9.030899869919436
gain= -9.030899869919436
gain= -9.030899869919436
gain= -9.542425094393248
gain= -9.542425094393248
gain= -9.542425094393248
gain= -9.542425094393248
gain= -9.542425094393248
gain= -9.542425094393248
gain= -9.542425094393248
gain= -9.542425094393248
gain= -9.542425094393248
gain= -9.542425094393248
gain= -10.0
gain= -10.0
gain= -10.0
gain= -10.0
gain= -10.0
gain= -10.0
gain= -10.0
gain= -10.0
gain= -10.0
gain= -10.0
gain= -10.41392685158225