Bài 2 :
`C=1/12+1/20+1/30+1/42+1/56+1/72+1/90+1/110+1/132`
`⇒C=1/3.4+1/4.5+1/5.6+1/6.7+1/7.8+1/8.9+1/9.10+1/10.11+1/11.12`
`⇒C=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10+1/10-1/11+1/11-1/12`
`⇒C=1/3-1/12`
`⇒C=4/12-1/12`
`⇒C=3/12`
`⇒C=1/4`