Chapter 9 Quadratic Relations and Conic Sections - 9.7 Solve Quadratic Systems - 9.7 Exercises - Problem Solving - Page 663: 44d

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We need at least three stations because if we only have two, we'll have two intersections between two circles, and there's no way of knowing which one is correct, so you'll need to add a third since then there would be only one point of intersection between all three circles.