2)
B $=1+\frac{7}{8}+\frac{23}{24}+\frac{47}{48}+\frac{79}{80}+\frac{119}{120}$
B $=1+1-\frac{1}{8}+1-\frac{1}{24}+1-\frac{1}{48}+1-\frac{1}{80}+1-\frac{1}{120}$
B $=6-(\frac{1}{8}+\frac{1}{24}+\frac{1}{48}+\frac{1}{80}+\frac{1}{120})$
B $=6-(\frac{1}{2\times4}+\frac{1}{4\times6}+\frac{1}{6\times8}+\frac{1}{8\times10}+\frac{1}{10\times12})$
B $=6-\frac{1}{2}\times(\frac{2}{2\times4}+\frac{2}{4\times6}+\frac{2}{6\times8}+\frac{2}{8\times10}+\frac{2}{10\times12})$
B $=6-\frac{1}{2}\times(\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{8}+\frac{1}{8}-\frac{1}{10}+\frac{1}{10}-\frac{1}{12})$
B $=6-\frac{1}{2}\times(\frac{1}{2}-\frac{1}{12})$
B $=6-\frac{1}{2}\times\frac{5}{12}$
B $=6-\frac{5}{24}$
B $=\frac{139}{24}$
Bài 1:
Tử số: $3+4+7+11+18+27+38+51+66+83+102+123$
$=(3+4+123)+(102+18)+(11+38+51)+(83+27)+66+7$
$=130+120+100+110+73$
$=533$
Mẫu số: $1,59\times31,4\times2+95,4:3\times6,86$
$=1,59\times2\times31,4+31,8\times6,86$
$=3,18\times31,4+3,18\times10\times6,86$
$=3,18\times31,4+3,18\times68,6$
$=3,18\times(31,4+68,6)$
$=3,18\times100$
$=318$
Vậy A $=\frac{533}{318}$
