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\int_{1}^{2} \mathrm{f}(x) \cdot \mathrm{g}(x) \mathrm{d} \mathrm{x} \neq \int_{1}^{2} \mathrm{f}(x) \mathrm{d} \mathrm{x} \cdot \int_{1}^{2} \mathrm{~g}(x) \mathrm{d} \mathrm{x}