Đáp án:
`1/(1xx2)+1/(2xx3)+1/(3xx4)+1/(4xx5)=4/5`
Giải thích các bước giải:
Cách `1`
`1/(1xx2)+1/(2xx3)+1/(3xx4)+1/(4xx5)`
`=1/1-1/2+1/2-1/3+1/3-1/4+1/4-1/5`
`=1/1-1/5`
`=5/5-1/5`
`=4/5`
Vậy `1/(1xx2)+1/(2xx3)+1/(3xx4)+1/(4xx5)=4/5`
Cách `2`
`1/(1xx2)+1/(2xx3)+1/(3xx4)+1/(4xx5)`
`=1/2+1/6+1/12+1/20`
`=3/6+1/6+1/12+1/20`
`=4/6+1/12+1/20`
`=2/3+1/12+1/20`
`=8/12+1/12+1/20`
`=9/12+1/20`
`=3/4+1/20`
`=15/20+1/20`
`=16/20`
`=4/5`
Vậy `1/(1xx2)+1/(2xx3)+1/(3xx4)+1/(4xx5)=4/5`
