`a)``A=2/1.5+2/5.9+2/9.13+...+2/33.37+2/37.41`

`=1/2.(4/1.5+4/5.9+4/9.13+...+4/33.37+4/37.41)`

`=1/2.(1-1/5+1/5-1/9+1/9-1/13+...+1/33-1/37+1/37-1/41)`

`=1/2.(1-1/41)`

`=1/2. 40/41`

`=20/41`

Vậy `A=20/41`

`b)``B=1/15+1/35+1/63+1/99+...+1/323`

`=1/3.5+1/5.7+1/7.9+1/9.11+...+1/17.19`

`=1/2.(2/3.5+2/5.7+2/7.9+2/9.11+...+2/17.19)`

`=1/2.(1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11+...+1/17-1/19)`

`=1/2.(1/3-1/19)`

`=1/2. 16/57`

`=8/57`

Vậy `B=8/57`

`c)``C=(112/13.20+112/20.27+112/27.34+...+112/62.69):(5/9.13+7/9.25+13/19.25+31/19.69)`

`=(112/13.20+112/20.27+112/27.34+...+112/62.69)/(5/9.13+7/9.25+13/19.25+31/19.69)`

`=(16.(7/13.20+7/20.27+7/27.34+...+7/62.69))/(2.(5/13.18+7/18.25+13/25.38+31/38.69))`

`=(16.(1/13-1/20+1/20-1/27+1/27-1/34+...+1/62-1/69))/(2.(1/13-1/18+1/18-1/25+1/25-1/38+1/38-1/69))`

`=(16.(1/13-1/69))/(2.(1/13-1/69))`

`=16/2`

`=8`

Vậy `C=8`