R=10;
L=.01;
ω0=314;
T=((2.Pi)/(ω0));
c=10*(10^(-6));
tmax=0.06;
uz[t_]:=10*Sin[ω0*t+0.13]+3*Sin[3.23*ω0*t+0.9]+5*Sin[9.23*ω0*t-0.9];
rce={uz[t]-u1[t]==R*i[t],u1[t]-u2[t]==L*i'[t],i[t]==c*u2'[t],i[0]==0,u2[0]==0};
nezname=Cases[rce,_Symbol[t],{0,∞}]//Union;
res=NDSolve[rce,{i,i',u1,u2},{t,0,tmax},StartingStepSize->(10^(-9)),MaxSteps->(10^6)][[1]];
Plot[i[t]/.res,{t,0,tmax}]

bod[t_]:={i[t],i'[t],i''[t],uz'[t]}/.res;
data1=Table[bod[t],{t,tmax-T,tmax,((tmax-T)/1000)}];
Fit[data1,{i,di,d2i},{i,di,d2i}]

derproud=Interpolation[{#,i'[#]/.res}&/@Range[tmax-T,tmax,((tmax-T)/1000)],InterpolationOrder->5];
bod[t_]:={i[t],derproud[t],derproud'[t],uz'[t]}/.res;
data1=Table[bod[t],{t,tmax-T,tmax,((tmax-T)/1000)}];
Fit[data1,{i,di,d2i},{i,di,d2i}]