arctanx 是 tanx 的反函數， 定義域值域互換，xy 互換
arctanx = 1 時， x=tan1
tan(π/4) = 1,
arctan1 = π/4 ,
tan1= ... = tan1
arctanx = 1
x= tan1
arctan tan1 = 1
arc tan 1 = π/4
1= tan π/4
x= π/4 , y =1 , tan π/4 =1
x=1 , y = π /4 , arctan 1 = π/4
x=... , y=1 , arctan ...=1
x=1 , y=... , tan1 =... = tan1
arctan... = arctan tan1 = 1 ✔