对于\(n\)个球，易得有\[\begin{array}{c}\displaystyle\frac\pi2>\theta_i>-\frac\pi2,\theta_1>\cdots>\theta_i>\cdots>\theta_{n-1}\\[1ex]\displaystyle\foralli\nej,\left\lvert\sum_{k=1}^ir_k-\sum_{k=1}^jr_k\right\rve

对于\(n\)个球，易得有\[\begin{array}{c}\displaystyle\frac\pi2>\theta_i>-\frac\pi2,\theta_1>\cdots>\theta_i>\cdots>\theta_{n-1}\\[1ex]\displaystyle\foralli\nej,\left\lvert\sum_{k=1}^ir_k-\sum_{k=1}^jr_k\right\rve