Four points of the center of four balls in space are purchased as a space quadrilateral, because all four balls are required to be circumscribed, that is, the side length of this space quadrilateral is the sum of the radii of two balls. To find a ball and every ball is circumscribed, we must find a point in this space quadrilateral and make its distance to each of the four vertices r+2, r+2, r+3 and r+3. This r is the radius of the ball to be found. Therefore, an equivalent model is established.

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