F(x)=A+Barctanx
F（-∞）＝A－Bπ/2=0
F（+∞）＝A+Bπ/2=1
A=1/2,B=1/π
F(x)=1/2+1/πarctanx
P（－1＜X＜1）＝1/π[π/4－(－π/4)]＝1/2
f(x)=F'(x)=1/π*1/(1+x^2)