The locus of the centre of a circle which touches the circles|z−z1|=a and |z−z2|=b externally is
an ellipse
a hyperbola
circle
none
|z−z1|=a+r,|z−z2|=b+r|z−z1|−|z−z2|=a−b
Locus is a hyperbola with foci at Z1&Z2