1.
If the circles x2+y2+2gx+2fy+c=0x^2+y^2+2gx+2fy+c=0 and x2+y2+2gx+2fy+c=0x^2+y^2+2g'x+2f'y+c'=0 are orthogonal, then 2gg+2ff=c+c+constant2gg'+2ff' = c+c' + \text{constant}. The constant is