A particle has co-ordinates (x, y). Its position vector makes on angle `theta` with positive x direction. In an infinitesimally small interval of time the particle moves such that length of its position vector does not change but angle `theta` increases by `d theta`. Express the change in position vector of the particle in terms of x, y, `d theta` and unit vectors `hati` and `hatj`.

Answer

`vec(Delta)r = (-y hati +x hatj) d theta`