回複：求救：有沒有好辦法?

In other words, the transformation will keep (a,b,c) on the same globe surface where the center is at (0,0,0). The biggest radius for the globe is 10 * 3^(1/2) = 17.32. That means, not only 20 is not reachable, it doesn't reach anywhere bigger than 17.32.