Let Cr x y R times R x2 y2 r2 where r R Findr Cr andr Cr
Let C_r = {(x, y) R times R | x^2 + y^2 = r^2} where r R^+. Find_r C_r and_r C_r. Prove your answer.
Solution
Cr represents the circle in the xy plane with centre at the origin and radius =r.
Since r belongs to r+, r cannot be negative
Intersection of all circles with radius in R+ will be the circle common to the family of circle with centre at origin.
Hence will be the point (0,0)
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Union of all circles will be the biggest circle in the xy plane with the radius r nearing infinity and centre at (0,0)
So union will be the full xy plane.
