要用到的公式：tan(A+B)=(tanA+tanB)/(1-tanAtanB)
tan(arctana)=a
所以有tan(arctanx+arctan1/x)
=(tanarctanx+tanarctan1/x)/(1-tanarctanx*tanarctan1/x)
=(x+1/x)/(1-x*1/x)
=(x+1/x)/0
=无穷大
=tanπ/2
x>0
00所以arctanx+arctan1/x=π/2成立