[tex]1.f=5 cm=0,05 m\\C=\frac{1}{f}= \frac{1}{0,05} =20\delta=>C=20\delta\\\frac{1}{x_2}- \frac{1}{x_1} =\frac{1}{f} <=>\frac{1}{x_2}=\frac{1}{f}+\frac{1}{x_1}=\frac{1}{5}-\frac{1}{30}=\frac{25}{150}= \frac{1}{6}=>x_2=6 cm\\ \\\\2.E_c=\frac{mv^2}{2}=\frac{4*20^2}{2}=2*400=800 J=>E_c=800 J[/tex]
