Đáp án:
$(1/2+5/6+11/2+19/20+29/30+41/42+55/56+71/72+89/90)×100$
$=(1-1/2+1-1/6+1-1/12+1-1/20+1-1/30+1-1/42+1-1/56+1-1/72+1-1/90)×100$
$=[9-(1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90)]×100$
$=[9-(1/1.2+1/2.3+1/3.4+1/4.5+1/5.6+1/6.7+1/7.8+1/8.9+1/9.10)]×100$
$=[9-(1-1/2+1/2-1/3+1/3-1/4+...+1/9-1/10)]×100$
$=[9-(1-1/10)]×100$
$=(9-9/10)×100$
$=(81/10)×100=810$
#NOCOPY