当x从右边趋于0时,1/x趋于正无穷,arctanx趋于π/2,所以此时
lim2(x+1)arctan1/x=2*π/2=π
当x从左边趋于0时,1/x趋于负无穷,arctanx趋于-π/2,所以此时
lim2(x+1)arctan1/x=2-*π/2=-π