A = | x - 1 | + | y + 3/4 | - 2020
Ta có : | x - 1 | ≥ 0 ∀ x ; | y + 3/4 | ≥ 0 ∀ y
=> | x - 1 | + | y + 3/4 | ≥ 0 ∀ x, y
=> | x - 1 | + | y + 3/4 | - 2020 ≥ -2020 ∀ x, y
Dấu "=" xảy ra <=> \(\hept{\begin{cases}x-1=0\\y+\frac{3}{4}=0\end{cases}}\Leftrightarrow\hept{\begin{cases}x=1\\y=-\frac{3}{4}\end{cases}}\)
=> MinA = -2020 <=> x = 1 ; y = -3/4