to find the plar of point (-1/2,1) with respect to ellipse   

 I do not understand what you mean by this question when you say "to find polar of point with respect to ellipse."

Polar cordiante of any point (x,y) are 

r = sqrt(x^2 + y^2) = sqrt(5)/2

and theta = taninverse(y/x) +pi = taninverse(-2) + pi (As x < 0)