9,1+10,2+11,3+...=16,8+17,9 / (1+1+2+3+5=..+55)-25
1/2*3=1/3*4+..=1/2010*2011
3 thung dau co 84 lit dau so dau thung 1 =3/5so dau thung 2 so dau thung 2=0,75% so dau thung 1 hoi moi thung ... lit ( trinh bay tri tiet)
bai 3;1 nguoi ban vai sau khi ban 40% tam vai roi lai ban50% so con lai thi con lai6m .hoi tam vai dai ...m( trinh bay chi tiet)