arctanx 表示一个角度,
a1=arctanx1;
a2=arctanx2;
tana1=x1;tana2=x2;
tan(arctanx1+arctanx2)=[tan(a1)+tan(a2)]/(1-tan(a1)*tan(a2))=
(x1+x2)/(1-x1*x2);
因此；
arctanx1+arctanx2=arctan[(x1+x2)/(1-x1*x2)];
而不是arcTanx1+arcTanx2=arcTan(x1+x2);