令a=arctan1/2∈(0,π/2)b=arctan(-2) ∈(-π/2,0)tana=1/2tanb=-2tanatanb=-1a-b∈(0, π)cot(a-b)=(1+tanatanb)/(tana-tanb)=(1-1)/(1/2+2)=0arctan1/2-arctan(-2) = a-b=π/2.
