t=var('t')
I=function('I',t)
DE = 0.5*diff(diff(I,t),t) + 40*diff(I,t) + 750*I - 25*100*cos(100*t)
I(t)=desolve (DE,[I,t])
I(0)==0
de(t)=diff(I,t)
de(0)==0
k1,k2=var('k1,k2')
solve([I(0)==0,
       de(0)==0],k1,k2)

print I.subs(k1=30, k2=-10)