`S=2/2.3 +2/3.4 +2/4.5 +...+ 2/199.200`
`S= 2.( 1/2.3 +1/3.4 +1/4.5 +...+ 1/199.200)`
`S= 2. ( 1/2 -1/3 +1/3 -1/4 +1/4 -1/5+...+1/199-1/200)`
`S= 2.[1/2+(1/3-1/3)+(1/4-1/4)+(1/5-1/5)+...+(1/199-1/199)-1/200]`
`S=2.(1/2+0+0+0+...+0-1/200)`
`S=2.(1/2 -1/200)`
`S=2.(100/200 -1/200)`
`S=2. 99/200`
`S= 198/200`
`S= 99/100`
chúc học tốt