Tìm GTNN :
A = |y-10|+35
B = |2x-16|-2021
C = |x^2-9|+2019
D = |x^2+1|+2020
Tìm GTNN : A = |y-10|+35 B = |2x-16|-2021 C = |x^2-9|+2019 D = |x^2+1|+2020
By Rose
By Rose
Tìm GTNN :
A = |y-10|+35
B = |2x-16|-2021
C = |x^2-9|+2019
D = |x^2+1|+2020
Đáp án:
`+)`
` A = ` l`y-10`l `+ 35`
Ta có
l`y-10`l `\geq 0` nên
` |y-10| + 35 \geq 35`
` => A \geq 35`
` =>` Min`A = 35` khi ` y – 10 = 0 => y = 10`
` +)`
`B = |2x-16|-2021`
Ta có ` |2x-16| \geq 0`
` => |2x-16| – 2021 \geq -2021`
` =>` Min `B= -2021` khi ` 2x – 16 = 0 => x = 8`
`+)`
`C = |x^2-9|+2019`
Ta có `|x^2-9| \geq 0`
`=>` l`x^2-9`l `+ 2019 \geq 2019`
` =>` Min `C = 2019` khi ` x^2 – 9 = 0 => x = ± 3`
`+)`
` D = |x^2+1|+2020`
Ta có `x^2 \geq 0 => x^2 + 1 \geq 1 => |x^2+1|\geq1`
` => |x^2+1|+2020 \geq 2021`
` =>` Min `D = 2021` khi ` x^2 + 1 = 1 => x = 0`
Giải thích các bước giải:
`A = |y-10|+35`
Ta thấy `|y-10|>=0`
`=>|y-10|+35>=35`
`=>A>=35`
Dấu `=` xảy ra `<=>y-10=0=>y=10`
Vậy $A_{min}=35$ `<=>x=10.`
`B = |2x-16|-2021`
Ta thấy `|2x-16|>=0`
`=>|2x-16|-2021>=-2021`
`=>B>=-2021`
Dấu `=` xảy ra `<=>2x-16=0=>2x=16=>x=8`
Vậy $B_{min}=-2021$ `<=>y=8.`
`C = |x^2-9|+2019`
Ta thấy `|x^2-9|>=0`
`=>|x^2-9|+2019>=2019`
`=>C>=2019`
Dấu `=` xảy ra `<=>x^2-9=0=>x^2=9=>x=+-3`
Vậy $C_{min}=2019$ `<=>x=+-3.`
`D = |x^2+1|+2020`
Ta thấy `x^2>=0`
`=>x^2+1>0`
`=>|x^2+1|=x^2+1`
`=>D=x^2+1+2020=x^2+2021`
Ta thấy `x^2>=0`
`=>x^2+2021>=2021`
`=>D>=2021`
Dấu `=` xảy ra `<=>x^2=0=>x=0`
Vậy $D_{min}=2021$ `<=>x=0.`