證明：?$(1)$?設(shè)這個(gè)四位數(shù)是?$\overline {abcd}，$?則?$\overline {abcd}=1000a + 100b + 10c + d$?
?$=(999a + 99b + 9c)+(a + b + c + d)$?
?$=3(333a + 33b + 3c)+(a + b + c + d)$?
?$ $?若?$a + b + c + d$?可以被?$3$?整除，∵?$3(333a + 33b + 3c)$?能被?$3$?整除
∴這個(gè)數(shù)?$\overline {abcd}$?可以被?$3$?整除
?$ (2) $?設(shè)?$y_{1} = x_{1}^2，$??$y_{2} = x_{2}^2，$?則?$y_{1} - y_{2} = x_{1}^2 - x_{2}^2=(x_{1} + x_{2})(x_{1} - x_{2})$?
?$ $?當(dāng)?$x_{1}＞x_{2}＞0$?時(shí)，?$x_{1} + x_{2}＞0，$??$x_{1} - x_{2}＞0$?
∴?$(x_{1} + x_{2})(x_{1} - x_{2})＞0，$?即?$y_{1} - y_{2}＞0，$?∴?$y_{1}＞y_{2}$?
