证明: $dps{int_0^{2pi}sex{int_x^{2pi}cfrac{sin t}{t} d t} d x=0}$.
证明: $$eex ea int_0^{2pi}sex{int_x^{2pi}cfrac{sin t}{t} d t} d x &=int_0^{2pi} int_0^t cfrac{sin t}{t} d x d t\ &=int_0^{2pi} cfrac{sin t}{t}cdot t d t\ &=int_0^{2pi}sin t d t\ &=0. eea eeex$$