It is well-know that the Sturm-Liouville theory justifies the "separation of variables"n method for voluminous partial differential equation problems. For Sturm-Liouville problems the Rayleigh quotient is  the basis of an important approximation method that is used in physics, as well as in engineering. Although any eigenvalue can be related to its eigenfunction by the Rayleigh quotient, this quotient cannot be used to determine the exact value of the eigenvalue since eigenfunction is unknown. However, interesting and significant results can be obtained fromthe Rayleigh quotient without solving the differential equation(i.e. even in the case when the eigenfunction is not known). For example, Rayleigh quotient can be quite useful in estimating the eigenvalue.  It is the purpose of this paper to extend and generalize such important spectral properties as  eigenfunction expansion and Parseval equality for Sturm-Liouville problems with interiorsingularities. We shall investigate certain spectral problems arising in the theory of the convergence of the eigenfunction expansion. Particularly, by modifying the Green's function method weshall extend and generalize such important spectral properties as Parseval's equality, Rayleigh quotient and Rayleigh-Ritz  formula for the considered problem.