7.7

$${\begin{array}{l} r_1 = 1 \cdot \frac {\hbar} {mv_1} \\ r_2 = 2 \cdot \frac {\hbar} {mv_2} \\ r_3 = 3 \cdot \frac {\hbar} {mv_3} \\ \cdots \\ r_n = n \cdot \frac {\hbar} {mv_n} \end{array} }$$