因为tan(arctanx)=x且 tan(x+y)=(tanx+tany)/(1-tanx*tany)于是有：tan(arctanx+arctany)=[ tan(arctanx)+tan(arctany) ] / [1 - tan(arctanx)*tan(arctany)]=(x+y)/(1-xy)于是对于arctan(4*w)+arctan(5*w)=5 两边同...