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The gap-to-Tc ratio (R) of high-temperature superconductors is calculated in the context of d symmetry of the superconducting order parameter and a two-dimensional Van Hove singularity in the density of states, using the BCS theory for weak coupling. Exact numerical calculation and an analytic formula for R are given. The ratios are found to be substantially larger than the BCS weak coupling limit of 3.53. The overall dependence of R on ωD/Tc, where ωD is the cutoff frequency, is given.