Đáp án + Giải thích các bước giải:
`B=1/3+1/3^2+1/3^3+...+1/3^100`
`=>3B=1+1/3+1/3^2+...+1/3^99`
`=>3B-B=(1+1/3+1/3^2+...+1/3^99)-(1/3+1/3^2+1/3^3+...+1/3^100)`
`=>2B=1-1/3^200`
`=>B=(1-1/3^100)/2`
`-------`
`C=2/3.5+2/5.7+2/7.9+...+2/103.105`
`=>C=1/3-1/5+1/5-1/7+1/7-1/9+...+1/103-1/105`
`=>C=1/3-1/105`
`=>C=35/105-1/105`
`=>C=34/105`
`-------`
`D=75%+5,1(1/3-2 5/6)-(1/3)^2`
`=>D=3/4+51/10(1/3-17/6)-1/9`
`=>D=3/4+51/10(2/6-17/6)-1/9`
`=>D=3/4+51/10. (-5)/2-1/9`
`=>D=3/4+(-51)/4-1/9`
`=>D=-12-1/9`
`=>D=(-108)/9-1/9`
`=>D=(-109)/9`
