1) = 28а + 42 - 12а = (28а - 12а) + 42 = 16а + 42
2) = 8х - 64 + 8х = (8х + 8х) - 64 = 16х - 64
3) = 1,7а - 6,8 + 3,6 - 1,2а = (1,7а - 1,2а) + (3,6 - 6,8) = 0,5а - 3,2
4) 12х - 9у - 12у + 7,2х = (12х + 7,2х) + (- 9у - 12у) = 19,2х - 21у
5) = - 4,3х + 2,4 - 5,8 + 2,6х = (2,6х - 4,3х) + (2,4 - 5,8) = - 1,7х - 3,4
6) = 8/15 * 15/4m - 8/15 * 5/16n - 3/20 * 20/3m + 3/20 * 40/9n = 8/4m - 1/6n - m + 2/3n = (2m - m) + (2/3n - 1/6n) = m + (4/6n - 1/6n) = m + 1/2n