Cho a >b. chứng minh rằng : 1) 1-3a < 1- 3b 2) 6a-5 > -5+6b 3) -3a-4 < -3b-4 4) -1+2a > -1+2b 5) -2a+3 < -2b+5 6) 4a+1 > 4b-7