a) 2/3: (1/2-3x) = -5/6
1/2- 3x= 2/3: -5/6
1/2- 3x= -4/5
3x= 1/2- (-4/5)
3x= 13/10
x= 13/10: 3
x= 13/30
b) 1/1.2+ 1/2.3+ 1/3.4+ 1/4.5+...+1/99.100
= 1- 1/2+ 1/2 - 1/3+ 1/3- 1/4+1/4- 1/5+...+1/99- 1/100
= 1- 1/100
=99/100
c)1/1.6+ 1/6.11+ 1/11.16+ 1/16.21+...+1/496.501
= 1- 1/6+1/6-1/11+...+1/496- 1/501
= 1- 1/501
= 500/501 nhưng do số thứ 100 có dạng 1/496. 501 nên bằng 1/5. 500/501
= 100/501
d) 8/1.5+ 8/5.9+ 8/9.13+...+ 8/41.45
= 8. ( 1/1.5+ 1/5.9+ 1/9.13+...+1/41.45)
= 2. ( 4/1.5+4/5.9+ 4/9.13+...+4/41.45)
= 2. ( 1- 1/5+ 1/5- 1/9+ 1/9- 1/13+...+ 1/41- 1/45)
= 2. ( 1- 1/45)
= 2. 44/45
= 88/45
e) S= 1/30+ 1/42+ 1/56+ 1/72+ 1/90+ 1/132
S= ( 1/5.6+ 1/6.7+ 1/7.8+ 1/8.9+ 1/9.10)+ 1/132
S= ( 1/5- 1/6+ 1/6- 1/7+ 1/7- 1/8+ 1/8- 1/9+ 1/9- 1/10)+ 1/132
S= ( 1/5- 1/10)+ 1/132
S= 1/10+ 1/132
S= 132/1320+ 10/1320
S= 71/660
CTLHN PLEASE ⭐
