[tex]d=|x_1|+x_2\\\beta=-\frac{x_2}{x_1}=>x_2=-x_1\beta=-4x_1\\d=|x_1|-4x_1=-5x_1=>x_1=-\frac{d}{5}=-\frac{60}{5}=-12 cm=>x_1=-12 cm\\x_2=-4x_1=-4(-12)=48 cm=>x_2=48 cm\\\frac{1}{x_2} - \frac{1}{x_1}=\frac{1}{f} <=>\frac{1}{48} +\frac{1}{12}=\frac{1}{f}<=> f=9,6 cm[/tex]
