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We consider the Feigin-Fuchs-Wallach-Rocha-Carridi free resolution for computing the cohomology of the Lie algebra of polynomial vector fields on real line. This construction requires the explicit formulae for some families of Virasoro singular vectors. The formula for one family has been already obtained by Benoit-St-Aubin. The formula for another family of Virasoro singular vectors has been obtained and it gives the complete answer for the coboundary operator  of the Feigin-Fuchs-Wallach-Rocha-Carridi free resolution.