[xn+(7-x)m]/7=[xm+(3-x)n]/3两边都乘以213[xn+(7-x)m]=7[xm+(3-x)n]把系数乘进去3xn+3(7-x)m=7xm+7(3-x)n3xn+21m-3xm=7xm+21n-7xn合并同类项3xn+7xn-3xm-7xm=21n-21m10xn-10xm=21n-21m提取系数10x(n-m)=21(n-m)因为...