Let A be the point (3, 0) and circles with variable diameter AB touch the circle x2+y2=36 internally. Let the curve C be the locus of the point B. If the eccentricity of C is e, then 72e2 is equal to .
Answer
18
Solution
[figure]
Let M be the midpoint of AB (centre of the variable circle, radius AB2) and T the point of contact. Let A'(-3, 0).
Internal contact with x2+y2=36: OM=6- AB2
In BAA', O and M are midpoints of AA' and AB OM= A'B2
A'B2=6- AB2 BA+BA'=12 (constant >AA'=6)
Locus of B is an ellipse with foci ( 3, 0): 2a=12, ae=3
a=6, e= 12
72e2=72 14=18
Answer: 18