$\\$
Đặt `A=1/(1.2)+1/(2.3)+...+1/(197 . 198)+1/(198 . 199)`
`->A = 1 -1/2+1/2-1/3+...+1/197 - 1/198 + 1/198 - 1/199`
`->A=1 + (-1/2+1/2)+...+(-1/198 +1/198)-1/199`
`->A=1-1/199`
`->A=199/199-1/199`
`->A=198/199`
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`B=1/199 - 1/(199 . 198) - 1/(198 . 197) - ... - 1/(3.2)-1/(2.1)`
`->B=1/199 - (1/(199 . 198)+1/(198 . 197)+...+1/(3.2)+1/(2.1) )`
`->B=1/199 - (1/(1.2)+1/(2.3)+...+1/(197 . 198)+1/(198 . 199) )`
`->B=1/199 - 198/199`
`->B=(-197)/199`
Vậy `B=(-197)/199`
