Đáp án:3/5
1- 5/6 +7/12 - 9/20 +11/30 - 13/42 + 15/56 - 17/72 + 19/90
= 1- 2+3/ 2.3 + 3+4/3.4 - 4+5 /4.5 + ...+9+10/9.10
=1- 3/2.3-2/2.3 +3/3.4 - 5/3.4 - 4/4.5 + ... + 10/9.10 - 9/9.10
=1- 1/2 -1/3+1/3 +1/4 - 1/4 -1/5 + ... +1/9 - 1/10
=1- 1/2+ 1/10
= 3/5