∫(-1~1) c/根号(1-x²) dx
=c*arcsin(x)|(-1~1)
=c*(π/2-(-π/2))
=cπ
cπ=1
c=1/π
然後求落在(-1/2,1/2)的概率
∫(-1/2~1/2) c/根号(1-x²) dx
=c(arcsin(1/2)+arcsin(1/2))
=c(2*π/6)
=1/3