Let the coordinates of point B be (h,k).
The center of the circle with diameter AB is C1(2h+3,2k) and its radius is r1=21(h−3)2+k2.
The given circle is x2+y2=36, which has center C2(0,0) and radius r2=6.
Since the circles touch internally, the distance between their centers is equal to the difference of their radii:
C1C2=r2−r1
(2h+3)2+(2k)2=6−21(h−3)2+k2
Multiplying the entire equation by 2, we get:
(h+3)2+k2+(h−3)2+k2=12
This equation represents the locus of a point B(h,k) such that the sum of its distances from two fixed points S1(−3,0) and S2(3,0) is constant and equal to 12. This is the standard definition of an ellipse.
Thus, the foci of the ellipse are (±3,0) and the length of the major axis is 2a=12.
The distance between the foci is 2ae=6.
Substituting 2a=12, we get 12e=6⇒e=21.
We need to find the value of 72e2:
72e2=72(21)2=72×41=18
Answer: 18