`*` Đề hình như sai á, bạn xem lại được ko?
$\dfrac{x.20}{11}$`=2/1.3+2/5.7+2/7.9+2/9.11`
$\dfrac{x.20}{11}$`=2/3 + 2/35 + 2/63 + 2/99`
$\dfrac{x.20}{11}$`= 2310/3465 + 198/3465 + 110/3465 + 70/3465`
$\dfrac{x.20}{11}$`= 2688/3465`
$\dfrac{x.20}{11}$`= 128/165`
Ta thấy: `11 = 165: 15`
`=> x.20 = 128 : 15`
`=> x.20 = 8,5 _((3)`
`=> x = 8,5_((3)) : 20`
`=> x = 0,42_((6))`
Xin sửa đề: $\dfrac{x.20}{11}$`=2/1.3+2/5.7+2/7.9+2/9.11`
`->` $\dfrac{x.20}{11}$`=2/1.3+ 2/3.5 +2/5.7+2/7.9+2/9.11`
`=>` $\dfrac{x.20}{11}$`=` $\dfrac{3-1}{1.3}$ `+` $\dfrac{5-3}{3.5}$ `+` $\dfrac{7-5}{5.7}$ `+` $\dfrac{9-7}{7.9}$ `+` $\dfrac{11-9}{9.11}$
`=>` $\dfrac{x.20}{11}$`= 1- 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + 1/9 - 1/11`
`=>` $\dfrac{x.20}{11}$`= 1- 1/11`
`=> x . 20/11 = 10/11`
`=> x = 10/11 : 20/11`
`=> x = 10/11 . 11/20`
`=> x = 1/2`
$#Dyn.Shun$
$@Shun$~
