• $f(a)=f(b)=0⟹f(x)=f(x)-f(a)=f(x)-f(b)$
• $f(x)={\displaystyle {\int }_a^x{f}^{\prime }(t)\mathrm{d}t=-{\int }_x^b{f}^{\prime }(t)\mathrm{d}t⟹2f(x)={\int }_a^x{f}^{\prime }(t)\mathrm{d}t-{\int }_x^b{f}^{\prime }(t)\mathrm{d}t}$