Giúp mình với ạ cần gấp

`A=1/{1.2}+1/{2.3}+1/{3.4}+...+1/{9.10}`

`A=1-1/2+1/2-1/3+1/3-1/4+...+1/9-1/10`

`A=1-1/10`

`A=10/10-1/10`

`A=9/10`

`B=2/{1.3}+2/{3.5}+2/{5.7}+...+2/{19.21}`

`B=1-1/3+1/3-1/5+1/5-1/7+...+1/19-1/21`

`B=1-1/21`

`B=21/21-1/21`

`B=20/21`

`C=1+2+2^2+2^3+2^4+...+2^10`

`=>2C=2(1+2+2^2+2^3+2^4+...+2^10)`

`=>2C=2+2^2+2^3+2^4+2^5+...+2^11`

`=>2C-C=(2+2^2+2^3+2^4+2^5+...+2^11)-(1+2+2^2+2^3+2^4+...+2^10)`

`=>C=2^11-1`

`#BaoNgoc`