Double_t R=.5*TMath::Sqrt(4.*a[0]+a[1]*a[1]+a[2]*a[2]);
Double_t dx=x0-xr0;
if (dx*dx>=R*R) return kFALSE;
- Double_t dy=TMath::Sqrt(R*R-dx*dx); //sign!!
+ Double_t dy=TMath::Sqrt((R-dx)*(R+dx)); //sign!!
if (TMath::Abs(yr0+dy)>TMath::Abs(yr0-dy))
dy=-dy;
Double_t y0=yr0+dy;