Đáp án + Giải thích các bước giải:
`A=1/1*2+1/2*3+1/3*4+...+1/49*50`
`=1/1-1/2+1/2-1/3+1/3-1/4+...+1/49-1/50`
`=1/1-1/50`
`=50/50-1/50`
`=49/50`
`B=2/3*5+2/5*7+2/7*9+...+2/37*39`
`=1/3-``-1/5+1/5-1/7+1/7-1/9+...+1/37+1/39`
`=1/3-1/39`
`=13/39-1/39`
`=12/39=4/13`