`A = 4/5 + (1/5 - 1/7) × 2/7`
`A = 4/5 + 2/35 × 2/7`
`A = 4/5 + 4/245`
`A = 196/245 + 4/245`
`A = 200/245`
`A = 40/49`
$\\$
`Đ = 1/2 + 5/6 + 11/12 + 19/20 + 29/30 + 41/42 + 55/56 + 71/72 + 89/90`
`Đ = (1 - 1/2) + (1 - 1/6) + (1 - 1/12) + ... + (1 - 1/72) + (1 - 1/90)`
`Đ = 1 - 1/2 + 1 - 1/6 + 1 - 1/12 + ... + 1 - 1/72 + 1 - 1/90`
`Đ = (1 + 1 + 1+ ... + 1) - (1/2 + 1/6 + 1/12 + ... + 1/72 + 1/90)`
`Đ = 9 - (1/(1.2) + 1/(2.3) + 1/(3.4) + .... + 1/(8.9) + 1/(9.10))`
`Đ = 9 - (1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/8 - 1/9 + 1/9 - 1/10)`
`Đ = 9 - (1 - 1/10)`
`Đ = 9 - 9/10`
`= 81/10`