まちごうたorz。
∫{1/(1+tanx)}dx
=∫{cosx/(cosx+sinx)}dx
=(1/√2)∫{cosx/(sin(x+π/4))}dx
=(1/√2)∫{cos(x+π/4-π/4)/(sin(x+π/4))}dx
=(1/2)∫{(cos(x+π/4)+sin(x+π/4))/(sin(x+π/4))}dx
=(1/2)log|sin(x+π/4)|+x/2+C