tan[a+b]= [tana+tanb]/(1-tanatab)tan[a-b]= [tana-tanb]/(1+tanatab)tan(x/2+π/4)-tan（π/4-x/2）= [tan(x/2)+1]/[1-tan(x/2)] - [1-tan(x/2)]/[1+tan(x/2)] = {[tan(x/2)+1]^2-[1-tan(x/2)]^2}/[1-tan^2(x/2)] ...