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If the circle x² + y² + 6x - 2y + k = 0 bisects the circumference of the circle x² + y² + 2x - 6y - 15 = 0, find the value of k.
- 21
- -21
- 23
- -23
Correct answer: -23
Solution
The radical axis (difference of the equations) is the common chord; it must pass through the centre (-1, 3) of the second circle, giving k = -23.
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