If the circle \(S_1 = x^2 + y^2 + 2gx + 4y + 1 = 0\) bisects the circumference of circle \(x^2 + y^2 - 2x - 3 = 0\), then the radius of circle \(S_1\) is
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Use geometric constraints like bisection to infer positional relation and derive radius.
If it bisects circumference, then it must pass through extremities of diameter of given circle. Use this and center-radius form to get radius of \(S_1\)