The two conics y2b2−x2a2=1 and y2=−bax intersect iff
0<a≤12
0<b≤12
b2>a2
b2<a2
The x co-ordinates of the points of intersection are given by x2a2+1abx+1=0
And the roots are real iff 1a2b2−4a2≥0⇒1b2−4≥0
⇒0<b2≤14⇒0<b≤12