Question

The condition for two circles S ≡ x² + y² + 2gx + 2fy + c = 0 and S' =  x² + y² + 2ga(x) + 2f'y + c' = 0 to cut orthogonally is that 2gg' + 2ff' + c + c'.

Answer 1

The two circles cut each other orthogonally