Đáp án:
`A=19/16`
Giải thích các bước giải:
`A=5/3.4+5/4.6+5/5.8+5/6.10+...+5/40.78`
`2A=5.2/3.4+5.2/4.6+5.2/5.8+5.2/6.10+...+5.2/40.78`
`2A=5/2.3+5/3.4+5/4.5+5/5.6+...+5/39.40`
`2A=5(1/2.3+1/3.4+1/4.5+1/5.6+...+1/39.40)`
`2A=5(1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+...+1/39-1/40)`
`2A=5(1/2-1/40)`
`2A=5*19/40`
`2A=19/8`
`A=19/16`