x=1/tdx= -(1/t^2) dtx=0, t=+∞x=2, t=1/2∫ (0->2) dx/(x^2+1)=∫ (+∞->1/2) -(1/t^2) dt/[ (1/t) ^2+1]=-∫ (+∞->1/2) dt/(t^2+1)ans : D
