câu 20,21 cứu djdsasnz

` Bài 20`

`A=-5/7-(-5/67)+13/30+1/2+(-1 5/6)+1 3/14 - (-2/5)`

`A=-5/7+5/67+13/30+1/2-11/6+17/14+2/5`

`A=(-5/7+1/2+17/14)+5/67+(13/30+1/2-11/6+2/5)`

`A=(-10/14+7/14+17/14)+5/67+(13/30+15/30-55/30+24/60)`

`A=1+5/67+(-30)/60`

`A=1+5/67-1/2`

`A=1/2+5/67`

`A=-77/134`

`Bài 21`

`A=(-1/2)+(-1/9)-(-3/5)+1/2006-(-2/7)-7/18+4/35`

`A=-1/2-1/9+3/5+1/2006+2/7-7/18+4/35`

`A=(-1/2-1/9-7/18)+1/2006+(3/5+2/7+4/35)`

`A=-(9/18+2/18+7/18)+1/2006+(21/35+10/35+4/35)`

`A=-1+1/2006+1`

`A=1/2006`