What is the arctangent of 1 ?
arctan 1 = ?
The arctangent is the inverse tangent function.
Since
tan π/4 = tan 45º = 1
The arctangent of 1 is equal to the inverse tangent function of 1, which is equal to π/4 radians or 45 degrees:
arctan 1 = tan-1 1 = π/4 rad = 45º