∫f(x)dx=∫[ln(1+e^x)/e^x]dx
=-ln(1+e^x)/e^x+∫dx/(1+e^x)
=-ln(1+e^x)/e^x-∫d(1+1/e^x)/(1+1/e^x)
=-ln(1+e^x)/e^x-ln(1+1/e^x)+C (C是积分常数)