Cho A = $\frac{1}{2}$ + $\frac{1}{3}$ + $\frac{1}{4}$ + $\frac{1}{5}$ + ... + $\frac{1}{308}$ $\frac{1}{309}$ và B = $\frac{308}{1}$ + $\frac{307}{2}$ + $\frac{306}{3}$ + ... + $\frac{3}{306}$ + $\frac{2}{307}$ + $\frac{1}{308}$ Tính $\frac{A}{B}$ Help em với anh cj ơi :((

`A=1/2+1/3+1/4+1/5+...+1/308+1/309`

`B=308/1+307/2+306/3+...+3/306+2/307+1/308`

`B=(309-1)/1+(309-2)/2+(309-3)/3+...+(309-306)/(306)+(309-307)/307+(309-308)/308+(309-309)/309`

`B=309/1+(309/2+309/3+...+309/306+309/307+309/308+309/309)-(1/1+2/2+3/3+...+306/606+307/307+308/308+309/309)`

`B=309+309(1/2+1/3+...+1/306+1/307+1/308+1/309)-309`

`B=309(1/2+1/3+...+1/306+1/307+1/308+1/309)`

`->A/B=1/309`