Circle F2: (x- 1)? +y? =9, center F2 (1, 0), radius r2=3.
The center of a moving circle is p and the radius is r.
* enclose ∴PF 1= 1+r
Endotangent ∴PF2=3-r
Pf1+pf2 =1+r+3-r = 4 = 2a (fixed value)
According to the first definition of ellipse, the locus of the center of a moving circle is ellipse a=2, C =1; b? =3
Ellipse p: x? /4+y? /3= 1