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Questions  

 If two circles (x+4)2+y2=1 and (x4)2+y2=9 are  touched externally by a variable circle, then locus of centre of variable circle is

a
x215−y21=1
b
x24−y212=1
c
x212−y24=1
d
x21−y215=1

detailed solution

Correct option is D

CS=r+1CS′=r+3CS−CS′=2Locus of C is hyperbola with (-4,0) and (4, 0) as foci. Here 2a=2, so a=1 and 2ae=8, so ae=4b2=a2e2−1=15

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