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1) The output of an RLC at resonance is indeed sinusoidal , The input of an LC circuit is always sinusoidal since there is no loss of energy.
2) It is not ideal in oscillators since the LC circuit has parasitic losses and the active part should add the energy lost, thus it is perfectly sinusoidal.
most oscillators have feedback and they oscillate only when they have positive feedback and in most cases this is satisfied at a particular frequency and we obtain sinusoidal output for such oscillators.... in some the delay between the output and input is used to make the circuit oscillate and in those circuits we get square wave output... the output is not necessarily square or sine... even triangle wave outputs can be obtained also we can superimpose of oscillators and obtain different waveforms...
As long as oscillator works in it's linear region the output will be allmost sinusoidal. The degree of feedback determines the oscillator amplitude and thus the waveform of output voltage. To keep the constant amplitude and thus a sine waveform a regulated feedback is neccessary.