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# The centres of a set of circles each of radius 3 lie on the circle x2+y2=25. The locus of any point in the set is

Arun
25763 Points
3 years ago
Dear student

Let C be the set of center points described by that equation x^2 + y^2 = 25. C is a circle of radius 5 centered at the origin.
Let S be the set of radius 3 circles with centers in C.
A point P is therefore on some circle in S if and only if it is exactly 3 units from some point in C.
A little sketching of that should convince you that the distance of P from the origin must be at least 5-3 = 2 and at most 5+3 = 8; and that any such point is in fact 3 away from some point on C and therefore part of some circle in S.
So, the points in you locus are the points (x,y) where:
2 ≤ √(x² + y²) ≤ 8
4 ≤ x² + y² ≤ 64 ... written without square roots

Regards