1) `A= 7/10.11 + 7/11.12 + 7/12.13 +...+ 7/69.70`
`A= 7 ( 1/10.11 + 7/11.12 + 1/12.13 +...+ 1/69.70)`
`A = 7( 1/10 - 1/11 + 1/11 - 1/12+...+ 1/69 - 1/70)`
`A= 7 ( 1/10 - 1/70)`
`A= 7. 3/35`
`A = 3/5`
Vậy `A= 3/5`
2) `B= 81 . [ ( 12 - 12/7 -12/289 -12/85)/(4 - 4/7 - 4/289 - 4/85) : (5+ 5/13 + 5/169 + 5/91)/(6 + 6/13 + 6/169 + 6/91) ] . 158158158/711711711`
`B= 81. [ (12(1 - 1/7 - 1/289 - 1/85))/(4 ( 1 - 1/7 - 1/289 - 1/85)) : (5(1 + 1/13 + 1/169 + 1/91))/(6( 1 + 1/13 +1/169 + 1/91)) ]. 2/9`
`B= 81 . 3 : 5/6 . 2/9`
`B= 81 . 3 : 5/6 . 2/9`
`B=324/5`
Vậy `B= 324/5`
3) `C = 1 -(12 + 12/19 -12/37 - 12/53)/(7 + 7/17+7/19 + 7/2012) : (3 + 3/19 -3/37 - 3/53)/(5 + 5/17 + 5/19 + 5/2012)`
`C= 1 - (12 + 12/19 -12/37 - 12/53)/(7 + 7/17+7/19 + 7/2012) . (5 + 5/17 + 5/19 + 5/2012)/(3 + 3/19 -3/37 - 3/53)`
`C= 1 - (12 + 12/19 -12/37 - 12/53)/(3 + 3/19 -3/37 - 3/53) . (5 + 5/17 + 5/19 + 5/2012)/(7 + 7/17+7/19 + 7/2012)`
`C = 1 - (12( 1 + 1/19 - 1/37 - 1/53 ))/(3( 1 + 1/19 - 1/37 - 1/53)) . (5(1 + 1/17 + 1/19 + 1/2012))/(7( 1 + 1/17 + 1/19 + 1/2012))`
`C = 1 - 12/3 . 5/7`
`C = -13/7`
Vậy `C = -13/7`
4) `D= (1 + 1/3 + 1/9 + 1/27)/(2 + 2/3 + 2/9+ 2/27) : (4 + 4/7 + 4/49 - 4/343)/(1 + 1/7 + 1/49 - 1/343)`
`D =(1 + 1/3 + 1/9+ 1/27)/( 2(1 + 1/3 + 1/9 + 1/27)) : (4(1 + 1/7 + 1/49 - 1/343))/(1 + 1/7 + 1/49 - 1/343)`
`D =1/2 : 4/1`
`D= 1/8`
Vậy `D= 1/8`
