Đáp án:
`a)`
` A =` l`x+2`l ` +2019`
Ta có
l`x+2`l `\geq 0`
` =>` l`x+2`l ` +2019 \geq 2019`
` => A_{min} = 2019` ; khi ` x +2=0 => x = -2`
`b)`
l`x+1`l ` \geq 0`
` => 2018-` l`x+1`l `\leq 2018`
` => B_{max} = 2018` ;
Khi ` x +1 = 0 => x = -1`
`c)`
Ta có
l`x-3`l `\geq0`;
l`y-2`l `\geq 0`
` =>` l`x-3`l + l`y-2`l `+2020 \geq 2020`
` => C_{min} = 2020` ; khi ` x -3=0 ; y - 2 = 0`
` => x = 3 ; y = 2`