$\\$
`a,`
`A = (x+3/7)^2 + 10/11`
Vì `(x+3/7)^2 ≥0∀x`
`-> (x+3/7)^2 + 10/11 ≥10/11∀x`
`->A ≥ 10/11 ∀x`
Dấu "`=`" xảy ra khi :
`(x+3/7)^2=0 ↔ x+3/7=0 ↔x=(-3)/7`
Vậy `min A=10/11 ↔x=(-3)/7`
$\\$
`b,`
`B=(x-4/5)^4 + 5/6`
Vì `(x-4/5)^4 ≥0∀x`
`-> (x-4/5)^4 + 5/6 ≥ 5/6 ∀x`
`->B ≥ 5/6 ∀x`
Dấu "`=`" xảy ra khi :
`(x-4/5)^4=0 ↔ x-4/5=0 ↔x=4/5`
Vậy `min B=5/6 ↔x=4/5`
$\\$
`c,`
`C=(x+1/2)^6 - 8/9`
Vì `(x+1/2)^6 ≥0∀x`
`-> (x+1/2)^6 -8/9 ≥ (-8)/9 ∀x`
`->C ≥ (-8)/9 ∀x`
Dấu "`=`" xảy ra khi :
`(x+1/2)^6=0 ↔ x+1/2=0 ↔x=(-1)/2`
Vậy `min C=(-8)/9 ↔x=(-1)/2`
