>>802
半角の公式から、tan^2(x/2)=(1-cos(x))/(1+cos(x))=t^2
→ cos(x)=(1-t^2)/(1+t^2、sin(x)=2t/(1+t^2)
tan(x/2)=t→dx=2*cos^2(x/2)dt=1+cos(x)dt=2/(1+t^2)dt