7) ((с2+с3+с4)*с1)/((с2+с3+с4)+с1)=6мкФ
8) I=P/U=1800/220=8.182A
R=U/I=220/8.182=26.88ОМ
Q=(U^2/R)*t=220*200*(25*60)/26.88=2.7MДж
14)((((R1+R2)/(R1*R2))+R3)*R4)/((((R1+R2)/(R1*R2))+R3)+R4)
=5.546Ом
Ir4=U/R4=3 A
Ir3=U/(((R1+R2)/(R1*R2))+R3)=29.455 A
ur3=Ir3*R3=29.455*6=146.73 B
ur1r2=U-ur3=180-146.73=33.27 B
Ir1=ur1r2/R1=33.27/90=0.37A
Ir2=ur1r2/R2=33.27/10=3.327