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\int_{\mathrm{a}}^{\mathrm{b}} \mathrm{f}(x) \mathrm{d} x=\int_{\mathrm{a}}^{\mathrm{b}} \mathrm{f}(t) \mathrm{d} t \quad=\int_{\mathrm{a}}^{\mathrm{b}} \mathrm{f}(w) \mathrm{d} w=\int_{\mathrm{a}}^{\mathrm{b}} \mathrm{f}(z) \mathrm{d} z