`(m + 20,21) + (m + 20,21) + (m + 20,21) + (n + 20,21) + (n + 20,21) + (n + 20,21)`
`= [(m + 20,21) + (m + 20,21) + (m + 20,21)] + [(n + 20,21) + (n + 20,21) + (n + 20,21)]`
`= 3 × (m + 20,21) + 3 × (n + 20,21)`
`Thay m = 20,21 - n`
`= 3 × (20,21 - n + 20,21) + 3 × (n + 20,21)`
`= 3 × (20,21 - n + 20,21 + n + 20,21)`
`= 3 × 20,21 × 3`
`= 3 × 3 × 20,21`
`= 9 × 20,21`
`= 181,89`
`text(Vậy giá trị của)` `(m + 20,21) + (m + 20,21) + (m + 20,21) + (n + 20,21) + (n + 20,21) + (n + 20,21)` `: 181,89`
