`text{Ta có:}`
`13-8x-2x^2`
`=21-8-8x-2x^2`
`=21-(2x^2+8x+8)`
`=21-2(x^2+4x+4)`
`=21-2(x+2)^2.`
` Vì (x+2)^2 ≥ 0 ∀ x`
`⇒2(x+2)^2 ≥ 0 ∀x`
`⇒21-2(x+2)^2 ≤ 21 ∀ x`
`⇒E ≤ 21 ∀ x`
`text{Dấu"=" xảy ra}` `⇔2(x+2)^2=0`
`⇔(x+2)^2=0`
`⇔x+2=0`
`⇔x=-2.`
`text{Vậy Max E=21 khi x=-2.}`
Đáp án:
`E = 13 - 8 - 2x^2`
`= 5 - 2x^2 ≤ 5`
Dấu "=" xây ra
`<=> x = 0`
Vậy MaxE là `5 <=> x = 0`
Đề khác :
`E = 13 - 8x - 2x^2`
`= - (2x^2 + 8x - 13)`
`= - (2x^2 + 8x + 8 - 21)`
`= -2(x^2 + 4x + 4) + 21`
`= -2(x + 2)^2 + 21 ≤ 21`
`<=> x + 2 = 0`
`<=> x = -2`
Vậy MaxE là `21 <=> x = -2`
_____________
`min E = - ∞`
Giải thích các bước giải:
BCNN(180, 320 ) gấp mấy lần ƯCLN ( 180 , 320 )
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