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 $=\frac{533}{318}$