左边推导出右边
设 x=arctana，y=arctanb，则 tan(x-y) = (tanx-tany) / (1+tanxtany) = (a-b)/(1+ab)，所以 x-y=arctan[(a-b)/(1+ab)]，也即 arctana - arctanb = arctan[(a-b)/(1+ab)] 。
