根据题目,目测是这样1 加到 49,减掉所有3的倍数（等差,从1,2,3,4,5.49）Sn = n/2 [ a + l ]sn = 49 / 2 [ 1+49]sn = 1225 （等差,从3,6,9,12,.48）Sn = n/2 [ 2a + (n-1)d]sn = 16 / 2 [ 2 * 3 +(16-1)*3]sn = 408两...