\[\begin{aligned} \int_{A}(p f+q g) d s &=\int_{A}(p f+q g) d s^{+}-\int_{A}(p f+q g) d s^{-} \\ &=p \int_{A} f d s^{+}+q \int_{A} g d s^{+}-p \int_{A} f d s^{-}-q \int_{A} g d s^{-} \\ &=p \int_{A} f...∫A(pf+qg)ds=∫A(pf+qg)ds+−∫A(pf+qg)ds−=p∫Afds++q∫Agds+−p∫Afds−−q∫Agds−=p∫Afds+q∫Agds.∫Afids=∫Aifids=∫Aifidt+∫Aifidu=∫Afidt+∫Afidu.