Thank you very much. At the beginning I tried with wrong statement:
WolframAlpha["L*x'+R*x=Abs[sin(t)] and x(0)=0",
PodStates -> {"Step-by-step solution"}]
Then I tried with the right statement:
WolframAlpha["L*x'+R*x=Sqrt[(sin(t))^2] and x(0)=0",
PodStates -> {"Step-by-step solution"}]