1991 = 153 * 13 + 2 1991 * 1991 = (153 *13 + 2) (153 *13 + 2) = (153 *13)^2 + 2 * 2* (153*13) + 4 = 13 * k +4 所以1991 * 1991 /13 余数是 4.