I have been thinking about it for sometime but couldnt really get the answer. This is the progress I have made till now. E |ψ> = H |ψ> E <p|ψ> = <p|H|ψ> Now how to evaluate the number <p|H|ψ>? Although I can evaluate this number by introducng identity operator 1 = [itex]\int[/itex]|x><x| dx. But for it I need to solve schrodinger's equation in position basis which defeats the purpose (because we will have to evaluate the number <x|ψ>).