$A=\genfrac{}{}{0ex}{}{1}{1.3}+\genfrac{}{}{0ex}{}{1}{3.5}+\genfrac{}{}{0ex}{}{1}{5.7}+.....+\genfrac{}{}{0ex}{}{1}{2021.2023}$

$=\genfrac{}{}{0ex}{}{1}{2}.\left(\genfrac{}{}{0ex}{}{2}{1.3}+\genfrac{}{}{0ex}{}{2}{3.5}+\genfrac{}{}{0ex}{}{2}{5.7}+....+\genfrac{}{}{0ex}{}{2}{2021.2023}\right)$

$=\genfrac{}{}{0ex}{}{1}{2}.\left(1-\genfrac{}{}{0ex}{}{1}{3}+\genfrac{}{}{0ex}{}{1}{3}-\genfrac{}{}{0ex}{}{1}{5}+\genfrac{}{}{0ex}{}{1}{5}-\genfrac{}{}{0ex}{}{1}{7}+....+\genfrac{}{}{0ex}{}{1}{2021}-\genfrac{}{}{0ex}{}{1}{2023}\right)$

$=\genfrac{}{}{0ex}{}{1}{2}.\left(1-\genfrac{}{}{0ex}{}{1}{2023}\right)=\genfrac{}{}{0ex}{}{1}{2}.\genfrac{}{}{0ex}{}{2022}{2023}=\genfrac{}{}{0ex}{}{1011}{2023}$

$1/2+3/5(x-2)=1/5$

$=>3/5(x-2)=1/5-1/2$

$=>3/5(x-2)=2/10-5/10$

$=>3/5(x-2)=-3/10$

$=>x-2=-3/10:3/5$

$=>x-2=-3/10xx5/3$

$=>x-2=-1/2$

$=>x=-1/2+2$

$=>x=-1/2+4/2$

$=>x=3/2$

$-3/3\ast 15/13-3/7\ast 11/13-3/7$

$-1\ast 15/13-3/7\ast 11/13-3/7$

$=-3/7:3/7\ast 15/13-3/7\ast 11/13-3/7$

$=-3/7\ast 35/13-3/7\ast 11/13-3/7$

$=-(3/7\ast 35/13+3/7\ast 11/13+3/7)$

$=-3/7(35/13+11/13+1)$

$=-3/7\ast 59/13$

$=-177/91$

$A=2/15+2/35+2/63+...+2/339$

$=2/(3.5)+2/(5.7)+2/(7.9)+...+2/(19.21)$

$=1/3-1/5+1/5-1/7+1/7-1/9+...1/19-1/21$

$=1/3-1/21=7/21-1/21$

$=6/21=2/7$

10,98751=10,9=10,99=10,998

0,09877=0,1=0,99=0,999

0,[45]=0,5=0,44=0,455

123,4567=123,5=123,46=123,457

vì phải quy đồng thì mới ra được mẫu số là 10

suy ra 2x/10-1/2y=1/10

vì 2x-1=1 suy ra 2x =2 vạy x =1

vì mẫu số trừ mẫu số thì giữ nguyên suy ra 2y=10 vậy y=5