F（x）=A+BarctanxF(-∞)=A-B*π/2=0F(+∞)=A+B*π/2=1A=1/2,B=1/πF（x）=1/2+1/π*arctanxP(|X|<1)=F(1)-F(-1)=1/2f(x)=F'(x)=1/π*1/(1+x^2)
