PlanarAngle

PlanarAngle[p -> {q1, q2}] gives the angle between the half‐lines from p through q1 and q2.

• PlanarAngle[{q1, p, q2}] gives the angle at p formed by the triangle with vertex points p, q1, and q2.
• PlanarAngle[..., "spec"] gives the angle specified by "spec".

Examples

PlanarAngle[{0, 0} -> {{1, 0}, {0, 1}}]

PlanarAngle[{{1, 0}, {0, 0}, {0, 1}}]

Please visit the official Wolfram Language Reference for more details.